basic way to think of it, any restriction in the system results in head loss, which really takes away from the head (or pressure) supplied by the pump. this will result in a drop in flow rate. For some answers on actually setting up a system and the effects of different types of fittings (avoid T's and elbow fittings like the plague) check out
THIS article. should explain some of the basics for you. also, there were a couple of threads here very recently about dual pump setups, you might do a search for those.