I rarely use split as any station that is working a pile up is unlikely to hear my QRP signal, but have occasionally been successful in the past with the FT817. It should not be high up in the to do list, at least in my view.
Thinking aloud:
If split is checked, then create a dedicated slave virtual receiver linked to the master receiver selected. Or allow the user to link a receiver to another one already opened using a selection box or similar. The master could then be tuned to the tx frequency, while the slave is tuned to the receive frequency, or vice versa. PTT toggles between the receivers. In receive some may want to be able to hear the tx frequency in one ear, and the receive in the other to be able to work out where the station is listening. When split is unchecked the slave is deleted.
Alternative, perhaps easier and more intuitive for a user, when split checked, two frequency displays appear, one for tx, the other for receive, maybe with a default 5k offset to start from. Different mouse buttons, or shift/ctrl click to set the alternative frequency from the waterfall. Mouse scroll can change the digit hovered over in either frequency display. Similar to the A/B VFO used in most rigs. Check boxes could enable a user to choose to listen on either or both frequencies when not transmitting.
No doubt getting a nice interface is harder than coding something that just works.
Another thought - as most SSB appears to be channelised to 2.5kHz intervals, eg 14.200, 14.202.5, 14.205 etc, it would be handy to be able to set the mouse scroll wheel tuning (or encoder from midi controller..) to optionally jump in channels, or when clicking on the water fall to default to the closest "channel".
Briliant work, great software.