Considering dinosaur packs are likely to be a regular thing, I think this constraint really ought naught be there. The limiting factor should be the scientists skills themselves akin to Research as others have pointed out. I understand it was for balancing reasons that we don't want expeditions to be too expediate, but given they are also a huge cost sink as well I think we have plenty of balancing tools without preventing us from sending out multiple expeditions.
It's going to take too long to get a good number of species into our parks if we can only go on one expedition at a time.