You can get a dual tank selector from a truck similar to the donor (it switches the feed and return lines), but reliability might be an issue, mine never seemed to want to work right. I wouldn't tee the return into the feed line, it would might work, but I could see it causing problems that could drive you crazy as well.

Once you have the wire harness torn down, you might see that you can get enough length out of it without extending it. But if you can't, a van harness has tons of length built into it.