If rigging to IGFA rules, there may be no more than 4 inches between the eye of the leading and trailing hook. The trailing hook must also be concealed by wing material on the fly. To make sure that all of my rigs meet those standards, I pre-measure and cut the wire to consistently achieve maximum regulation lengths.
When selecting hooks, I try to go with a trailing hook that is one or two sizes smaller than the lead hook. This helps make the fly a little more aerodynamic when casting. It also relegates the trailing hook to a supporting role; that is, it will be the secondary point of contact for the fish, rather than the primary.
The type of hook you choose is determined by two factors: personal preference and target species. For most of my fishing, I prefer a straight shank hook for the lead and either a straight shank or upturned eye for the trailing hook. If there is a particular hook that you prefer for a specific species, by all means, use it.