It isn't any different than running them in parallel. Actually, it would be safer for the batteries if they were ran in parallel instead of series. Imagine you have a lake and you are trying to fill a swimming pool. With parallel wiring, it would be like running 2 hoses into the pool. With series, it would be like running a hose and connecting it to another hose to get it there.

As parallel, the batteries work separately and the one that can provide the electricity the easiest to the esc will do just that, whereas in series, they have to work together. The esc just pulls what is needed at the given moment so doing it either way is irrelevant in the end. If you really want to get technical, running 2) 14 volt batteries in parallel would be better than running 2 7 volt batteries in series.

Also, all lipos are made at 3.7 volts. They just series them internally to get to 7.4 (2s) 11.1 (3s) 14.8 (4s) etc....