| jbadams |
Familiar Melding allows you to possess the body of your familiar, effectively leaving your own body 'dead' in the meantime; so nothing would happen to your own body, and you would instead be controlling the familiar.
So, your familiar assumes the form of the gargoyle (assuming that's a valid creature type... I didn't check that), gaining the abilities listed under Monstrous Physique, and the duration of this is doubled thanks to Spell Sponge.
You then use Familiar Melding to possess your familiar, essentially acting as it instead of yourself with the exceptions (potentially being able to speak and cast spells, etc.) as listed in the description of Familiar Melding.
You definitely don't both become gargoyles, sorry. :)