Well, I would say that it all depends what the pressures are in each pipe.
For example the pressure on the return leg when it his the T, is going to cause flow disruption, which the pump with either suck forward under negative pressure at the pump, or petrol will be pushed again the flow. These types of pumps are always best placed close to the tank and as low as possible. They do draw fuel into the pump, but they are positive pressure pumps as opposed to suction pumps. If the pump pressure is high, say at idle there will be perhaps high pressure at the T (on the return leg) where the negative pressure is on it way to the pump. This will cause the pump to work harder and could encourage a form of cavitation at or around the T junction, depending on pressure and flow.
If, as you say, you can't return the return leg fuel to the tank at least buy a 'Y' shaped stainless connector so that the petrol coming from the tank and the return fuel (from the carbs etc) join the two short ends on the 'Y' and this will I think overcome any undesirable pressure problems - in my opinion.