I am anticipating the delievery of 2 aircraft 1 month apart. Is it possible to pre-plan the route assignments for them so that as soon as they are delivered they can commence service?
Yes.
If you ordered new planes, pre-delivery is possible approx 3wk before dlv. (and for used planes instantly after order is placed)
https://www.airwaysim.com/game/Manual/Aircraft/New/#Delivery