would be nice if it was possible, maybe some small restrictions like these:
host army: you must pick either
CSM or daemons to be the "host army"
army layout: for each unit type you include from the host army (elite, troop etc) you may include a unit from the sub army of the same type.
example: host army is daemons, you take 3 units of blood letters and a blood thirster.
you are then allowed to take 3 troop and 1
HQ choice from the
CSM list.
thats just a way to stop stupidly strong spam armies
also, you may want to add in a rule that only units with marks of the chosen gods may be taken.