Not quite. Everything is known well in advance, and the time for the actual computation may be the same for one and two res ops. However, after a result is computed it must be routed from the producer to all the consumers it will have in the next cycle. That route time is independent of which op produced the result. The route time is largely dependent on the number of sources, i.e. the number of distinct operands that can be dropped in each cycle, which is the fan-in to the routing crossbar.
We want to keep the routing time low, because every operation pays that cost. So we want few belt sources, and need to move low-value ops out of the way. We do that with the two-stage crossbar, and relegate all multi-result ops and all non-lat-1 ops to the second stage. There may be a two-res op that is fast enough that its result is available as fast as a simple add. But putting those two results on the fastpath would add a cost to every other fastpath op, including critical things like add.