You would connect the two positives and connect the two negatives to put the
batteries in parallel. That, in effect, becomes one 3.7 volt battery. The voltage would remain at 3.7, but should about double the run time. Just don't connect the positives to the negatives, if you know what I mean. Short time.
Stacking the
batteries end to end would double the voltage but not give you much longer, if any, run time.