Sorry to necro post, but this is another great script from you MOD. :-) If only it could work with previously saved games.
Like take the current party in the save file and make it
$game_parties[0] = $game_party
I am also having trouble merging thee parties. What I mean is this:
$game_parties.merge! (1,2)
$game_party=$game_parties[1]
$game_parties.merge! (0,1)
$game_party=$game_parties[0]
$game_player.refresh
Party 0 seems to become party 1 instead of a merge of them both. It merges the items, but deletes everyone from party 0 and merges to party 1