In my case midi-keyboard (mk) is connected to Midi In port of A4. This allows me to program sequencer from midi-keyboard and play manually during lives. Midi channel of mk is equal to global channel of A4 which allows to play sound from active track without changing of midi channels of mk. Midi Out port of A4 is connected to Midi In of OT. Also A4 sends clock, transport and program change commands to OT. I use A4 for melodic stuff (and drums in case I use A4 standalone) and OT for drums and fx only because I like to use transpose function ([transpose] + [note key on integrated keyboard] ) of A4 during lives. In future I plan to buy AR and probably sell my OT. Also I’m waiting implementation of transpose function via midi to connect 2 A4 synths and control pitch from one unit (smt like chaining). CV interface is very useful to connect and control additional synths (Moog Minitaur in my case) because transpose function also works for it.