It should be possible to use several MIDI controllers at the same time if the MIDI channel can be changed
on those controllers.
If two MIDI devices are set to the same MIDI channel, Daslight cannot differentiate them. It's like the devices were cloned.
As far as I know, the APC 20 and APC 40 do NOT have
a MIDI setup to change their channel. It's always channel 1.
If there is something to "blame", it's the lack of options on the APC's, not Daslight.
Unfortunately, it will be the same issue with any other software !