Before bridge 2 batteries together, ensure voltage difference between them less than 50mV. New batteries will very likely have about 3.9V voltage, and varies very little.

One more WARNING! Be very careful with B- test point. DO NOT MESS WITH THEM. B- are not protected by "Protection Circuits". Do NOT bridge B-, unless you know what you don't know!

Of course. NEVER short-circuit any B- to any B+, or P- to B+, or P- to B-!