Why is my toilet bubbling?
Pretty sure a vent pipe (the one going through the roof) is blocked.  There should be a vent pipe coming off the main drain stack, perhaps not obvious in a multi story home.  Birds and other critters are sometimes the cause

Vent blockage means air is reversing through the toilet trap, removing the protective water (which keeps sewer gas from entering home).  

Water emptying from tank may just mean a leaky flapper, though this is unlikely on 2 toilets simultaneously.

A visit from a plumber is in order.