Paul,
Don't sweat this. Just issue orders to the force you think will have the best chance of success against the enemy at the objective. All a complex attack really is, is one involving multiple HQs. So if you are going up against an enemy Bn then you are going to need more than one friendly Bn as a general rule. So just pick the bets force to hand, assign whatever extra support you think it may need and let the AI do the rest. If you prefer, and many on this forum do, then order your Bns indivudually. Eg order yourt Inf Bn to assault through the covered terrain, your armoured Bn to assault through the clear terrain and either set your arty to fire at call or do what I do and set half to on call and the other half assign in direct support of each of the attacking Bns. Try out both methods and see what works best for you. That's the beauty of the sandbox that is Command Ops. You can experiment. Just save before each different approach.