The Nord 2 does split but only with Organ. So this is an option I have considered but it limits me to an organ and organ rather than two different instruments such as organ and Rhodes or piano.
Right, and that's something that was addressed on the Electro 5 and 6 (as well as options from other brands). those let you split an organ and a piano or an organ and a Rhodes (though still not a piano and a rhodes, though other boards--including the Numa--let you do that as well).
The Numa seems to be the answer, my only reservation is that it does the whole MIDI keyboard link to a computer thing which I don't need and won't use. So I am just mulling over some of the other suggestions, but I expect I will get a Numa.
Pretty much any board that does everything someone wants will also do some things they don't care about. ;-) Numa is a good value, and is even one of the options that would let you split piano and organ on a single board. I'd say your Electro will be the better sounding organ and maybe also Rhodes, but you may still find the Numa sounds good enough, which would also allow you to travel with just one board instead of two; and with 88 keys total, not worry so much about running out of keys for one sound or the other.
Though if you will continue to be using the Nord regardless and are strictly looking at your new purchase as a second board (your original proposition), then you won't need organ sounds in your second keyboard, and therefore you could save some money by buying the Numa Compact 2 instead of the 2x. That said, even if you don't need your second board to have anything but acoustic and electric pianos, there are some potential advantages to the 2X for you:
... it does give you the option of being able to sometimes use just the one board for everything, even if that's not always what you want to do
... when using it in conjunction with the Electro as a lower manual for organ, you can set its 9 sliders to be drawbar controls
... you'd have the option of using the Numa for drawbar organ while using the Electro for a piano or Rhodes sound, in case you prefer the Nord's piano/EP for some songs (whether because of sound or action), as long as the Numa's organ sound is still good enough.
If none of those three things are important to you, then the cheaper Numa Compact 2 (and for that matter, some other boards as well) could become options.