Another factor to consider is cost impact on future construction. Almost anything can be done somehow, but if the pipes are close enough that a contractor has to modify his methods or take additional care, the cost impacts can be significant. One way you could keep the pipes close and minimize future costs would be to specify the first pipe be backfilled with Controlled Low Strength Material (CLSM, flowable fill) for all or or a portion of the trench depth. It generally costs a bit more than conventional methods of backfill and compaction because of its comparatively higher material cost, but that higher cost is usually considerably offset by savings in reduced equipment and labor costs. Additionally, the total cost of the CLSM option can be further reduced if the required trench width is reduced in the design. Often trench widths are wider than they need to be to allow space for the pipe and cushion, plus enough space for compaction equipment to compact between the pipe and trench wall. Using the CLSM, that width can be reduced so that sometimes it can actually cost less than more traditional methods that require a wider trench. If 6" of cushion material is adequate between the pipe and trench wall, that is plenty of room to place the backfill on the sides of the pipe. The benefit in this case is that when the excavation is performed for the second pipe, the side nearer the first pipe can excavated be through the CLSM, which will be stable and will stand vertically at a depth of four feet. Using this method, 5' apart o.c. is plenty, and will work in almost any soil condition.
