Quite a few people have dual return pumps and lines. I do. Since pumps fail fairly regularly, it really reduces risk to have the extra redundancy. But: If one pump dies or is turned off, remember that it can siphon back. And it siphons back to the return area of the sump, which may be separate from skimmers and such. That happened to me. One pump was happily pushing water from sump to DT. One pump was off, and siphoning almost as much water back from DT to sump. So a very limited amount of water was going over overflow and through skimmer. Something to think about when designing your setup.