Excellent question and I'm going to guess a little bit here BUT, head pressure is caused by the weight of the water pressing downwards onto the outlet of the pump. With a "T" in the return pipe, the weight of the water is increased because the pipes hold more water. The force of the water coming out of the pump is going to slam into the T and then be diverted in two directions. But the water in the upwards pipes will be pushing back.
Therefore, your first statement is probably correct in that each pump will have 6-7 feet of head pressure where as a single pump would have 13 feet of head pressure. In fact, the single pump would probably have more then 13 feet because of the T.
I can think of several excellent reasons for using two pumps. If one fails, the other is still providing flow through the tank and will give you some time to recover from the failed one. Another reason, you can unplug one, take it out for maintenance, and the other one is keeping flow moving.
Downside, you have two pumps to maintain, they would probably use more electric, they would probably add a bit more heat to the water then a single pump would.
Either solution would work and provide you an awesome system.