Originally Posted by cavie
This is different than 2 in series. That dogbone WILL NOT work on a GFI Circuit. Take it from a master electrician. GFI can not share a neutral with the opposite phase. Hot and neutral have to match. The neutral goes thru the GFI breaker, unlike a normal non-GFI circuit. The average person has no idea how a GFI works.
You are assuming that the two sources feeding the adapter are different phases. If they are then yes the GFCI will trip. I've been in more than one cg where I've brought to management's attention the improperly wired or worn out 30 Amp NEMA TT-30R, 2P, 3W outlet. Typically cg "maintenance" will show up to replace it. I've had to explain to some of them why it was incorrectly wired as well as why the outlet was defective even though the contacts still carry electricity.
For someone using one of these at home you have a 50/50 chance that the outlet that the 15 Amp side and the 30 Amp outlet are the same phase.