"deWaverley "'s post discusses the pro and cons correctly.
I would like to add two questions:
- what kind of organ do you want to play: theatre or church?
For VTPOs
two swells are almost a must ( control of the shades of two separate chambers with important influence on the sound "picture" and removing the RF from them under performance is quite seldom.
For most of the church organ music one swell will do, unless you intend to control also the crescendo feature, which IMHO is a control for advanced and ambitious organists.
- did you check your Alesis keyboards wether they convert the direct volume control of the instrument into MIDI?
This can be checked with a free software called Midi-Ox - if not known: it is a very usefull and mighty tool for your further work.
And: IMHO it would probably cause problems if a Fatar is plugged into the Alesis Volume control socket, what anyway would not control the shades through HW.
Rgds,
amun