Survival is built around only allowing you to be in one party. If this was implemented, I think it would be pretty much the same thing as having an alt account in a different party, which is against the rules. You can also get in trouble for transferring parties with your remaining /bal from your party before. If I'm not mistaken both of these things would be considered boosting which is bannable so I don't think implementing this would be a good choice or fair to people who have been banned for essentially the same thing you're asking for.