I went through this when I was going to run efi on my boat...single engine 2 tanks...the problem I saw with doing tees is that it will ultimately suck one tank dry and then suck air as opposed to sucking from the other tank and same with the return...if its sucking from one tank and returning to the other then you have a full or overflowing tank while its sucking the other one dry...I have since just stuck with carbs which don't need a return so the 3 way imco valve I have will work but I would go with the 6 way and be done...use 3 for the feed and 3 for the return..I also found that if you are handy with sheet metal at all you can buy the parts from imco to convert a 3 way to a 6 way for like 100-150 bucks and make a sheet metal piece to hold the new parts on the outside front of the existing valve...if you look up the drawings of the 3 way vs the 6 way you will see exactly what I'm talking about and its much much cheaper than buying the 6 way that is stupid expensive....that's just my experience and opinion

You could also achieve the same thing with manual valves if you want to open the hatch and switch them when you run one tank dry but I like to do it from a switch as I try to run the tanks down even as opposed to running one empty and then switching to keep the boat a little more level

When converting my boat to EFI (Holley HP) several years ago, I ran the return line thru a power steering cooler, then to a "T" fitting on the suction side of the lift pump. No need for a return to the tank.
Be sure to install both pumps as low, and as close to the tanks as possible.

I have two engines, a generator, and three tanks, the valving is complicated enough with out return lines.