I don't have any experience with the Duo feature, but I think there are differences between the Duo feature and the Split feature, although I can't tell you what all of the differences are.
You can set the Split Voice to the same Voice Number as the Main Voice, but you can't use a dual-layered sound for the Split Voice. As long as you've set the Main Voice to a single-layered sound-- that is, one which does not turn on the Dual feature and set the Dual Voice-- you can set the Split Voice to the same sound.
Also, the Split Voice doesn't have Attack, Release, Cutoff, and Resonance settings like the Main Voice does, so you won't be able to modify those settings on the left side of the Split Point; but that shouldn't matter unless you like to adjust those settings on the Main Voice.
I think the biggest challenge will be adjusting the Transpose, Main Octave, and Split Octave settings to get more or less the same range of notes on the left and right sides of the Split Point, because there are limits to how far you can shift the notes with those settings-- but I think you should be able to do this.
I don't know whether the Duo feature has any special functionalities besides the simple convenience of setting the same voices and having essentially the same ranges of notes on the left and right sides of the Split Point.
We already know that the PSR-EW410 doesn't have the Duo feature; the question is whether its Split feature can be used to mimic the Duo feature.
The answer is a conditional "Yes," depending on whether the Duo feature has any special functionalities aside from being a quick and simple way to set
the Split Point,
the Main and Split Voices and their various parameters, especially
the Main and Split Octaves, and
the Dual and Split on/off settings.
As long as that's all Duo does, and it has no special functionalities which are integrated with the onboard Y.E.S. lesson feature, then you can mimic it on the PSR-EW410 by doing the following:
(1) Turn on the PSR-EW410, or if it's already on and has been in use then press the PORTABLE GRAND button. The keyboard's settings will be initialized to their "Portable Grand" default values.
(2) Press the FUNCTION button to enter the Function menu, use the CATEGORY buttons to step through the various functions, and use the plus ("+") and minus ("-") buttons on the numeric keypad to make the following changes as needed.
(3) Transpose should be initialized to 00; if not, set it to 00.
(4) The Split Point should be initialized to 054 (F#2 in Yamaha's numbering); set it to 065 (F3).
(5) Note the Main Volume; it's initialized to 122 on my PSR-EW400, but might be initialized differently on the PSR-EW410.
(6) The Main Octave should be initialized to 0; set it to -2 (minus 2).
(7) The Main Pan should be initialized to 064; if not, set it to 064-- unless you want to shift it to the right as described below in #16.
(8) Note the Main Reverb and Main Chorus; they're initialized to 028 and 000 respectively on my PSR-EW400, but might be initialized differently on the PSR-EW410.
(9) The Main Attack, Main Release, Main Cutoff, and Main Resonance should all be initialized to 064; if not, set them all to 064.
(10) Set the Split Voice to 001 ("LiveGPno").
(11) Set the Split Volume to 122, or whatever the Main Volume is set to.
(12) Set the Split Octave to 1 (plus 1).
(13) The Split Pan should be initialized to 064; if not, set it to 064-- unless you want to shift it to the left as described below in #16.
(14) Set the Split Reverb to 028, or whatever the Main Reverb is set to.
(15) Set the Split Chorus to 000, or whatever the Main Chorus is set to.
(16) The sound of the keys is programmed to naturally shift a little bit to the left or right as you play the keyboard, with Middle C being in the middle, to simulate how the sound of each key-- as produced by a hammer hitting a string-- follows its position on the keyboard. Setting the Main Pan and Split Pan to 064 should make both sides of the Split Point sound as though they're coming from exactly the same positions. However, if you'd like it to sound as though the left side is coming more from the left and the right side is coming more from the right, you can increase the Main Pan and decrease the Split Pan as desired. For instance, I think setting the Main Pan to 088 should shift the sound of the Main Voice two octaves to the right (064 + 024 = 088), and setting the Split Pan to 052 should shift the sound of the Split Voice one octave to the left (064 - 012 = 052). I'm not certain whether the Pan values reflect semitones as I've assumed, but I tried those settings on my PSR-EW400 and they seem to work perfectly.
(17) The Dual feature should be initialized to "off"; if not, press the DUAL button to turn it off.
(18) The Split feature should be initialized to "off"; press the SPLIT button to turn it on.
(19) Play the keyboard to see how you like the results of the above settings, and make any modifications to them as desired.
(20) Since you probably don't want to go through the above procedure each time you want to mimic the Duo feature, you'll probably want to save everything to a Registration Memory once you're satisfied with the results. Assuming you want to save the settings to the very first Registration-- Bank 1, Memory 1-- press the BANK button and set the Bank to 1 if it isn't already. Then press and hold down the BANK button and press the REGISTRATION MEMORY 1 button to write all of the current settings to the Registration.
(21) Now for the test. Turn off the keyboard and wait for several seconds to be sure it's completely off (meaning no residual power is flowing through it), then turn it back on. Press the REGISTRATION MEMORY 1 button by itself to recall the Registration, then play the keyboard to verify that everything sounds the way you want it to. When you're ready to reset the keyboard back to its normal default settings, press the PORTABLE GRAND button.
The PSR-EW410 has 76 keys, ranging from E0 to G6 in Yamaha's numbering. (In the correct Scientific Pitch Notation numbering, they actually range from E1 to G7, but Yamaha refers to Middle C as C3 rather than C4.) Setting the Split Point to 065 will split the keyboard exactly in half-- 38 keys on the left ranging from E1 to F4 in Yamaha's numbering, and 38 keys on the right ranging from F#1 to G4 in Yamaha's numbering. Middle C on the left side of the Split Point will be the second C from the left, which is normally the C one octave below Middle C; and Middle C on the right side of the Split Point will be the second C from the right, which is normally the C two octaves above Middle C. Thus, the left and right sides of the keyboard will both be able to play one full octave below Middle C and one full octave above Middle C, plus several additional keys to the left and right of those octaves.