I forgot to mention that one possible wrinkle with using one manual (whether upper or lower) to control a sound module app and the other manual to play the Orla itself is that you might have trouble preventing the Orla from generating its own sounds when you're playing on whichever manual you want to use as a controller, resulting in sounds from both at once.
Most MIDI-enabled keyboard instruments have a "Local Control" setting that essentially lets you disconnect the keyboard's manual(s) from its internal sound generators, such that playing on the keys will no longer produce any sounds from the instrument, so you can use the keyboard to play an external sound module without also getting the keyboard's internally-generated sounds. Basically, with "Local Control On" you get the keyboard's internal sounds, and with "Local Control Off" you get no sounds from the keyboard-- at least, not normally.
The problem with that is, Local Control is usually implemented on a system-wide basis, rather than on a channel basis, meaning you can turn it on or off for the entire keyboard, but you can't selectively turn it on or off just for specific channels (even though the official MIDI documentation for the Local Control message seems to indicate that it can target either specific channels or all channels).
The workaround for that hiccup is to turn off Local Control, send the keyboard's MIDI to a computer or MIDI box, filter and/or modify the MIDI as desired, and then route some of the channels back to the keyboard as desired and route the other channels to other sound modules as desired. In essence, you're using the keyboard's manual as though it were a controller to play the keyboard's sound generators as though it were a sound module. This might sound like a lot of hoops to jump through, but it might actually let you get more functionality out of your keyboard than its panel controls otherwise permit-- something for another thread, and which has already been discussed in other threads, so if you're interested in learning more then you might want to search for those threads.
Note that the MIDI settings on some keyboard instruments are advanced enough that they let you do those sorts of things without needing to use the Local Control setting at all. They let you select which keyboard parts you want to transmit and which MIDI OUT channels you want them to be transmitted on, as well as which MIDI IN channels you want to allow and which parts you want them to affect.
But for keyboards whose MIDI settings don't offer that level of sophistication, a combination of using Local Control and using a MIDI event filter/processor/router can work wonders.
