The GFCI looks at its own load, and if they're different by more than 5ma (for the household units) they trip. My GFCI tester (home built) draws 8 ma from the hot lead to ground. Putting two in series shouldn't be a problem, unless one of them is faulty. And these things do get squirrley - particularly the household GFCI breakers that go into the panel. These things are $40 a shot, and go bad pretty frequently.