If this was an actual "group content" that people can enjoy together, like Alice or CT, then I would be opposed to this suggestion.
But that's not the case, at this point its just an outdated instance where the basic instance rewards are completely useless. It's not like other group contents like Alice/CT etc where everyone earns a good reward for participating. The only valuable reward from this instance is the MVP card, which is not a good motivator for recruiting a party at all: Why would anyone want to make parties when the only possible reward is a card that they can't share? Obviously everyone prefers dual clienting to join so that they'll actually have the card to themselves, and have 2x more chance of obtaining it.
I'm against gameplay mechanics giving an advantage to multi-client over single-client, or forcing people to multic-client, so obviously I hate the current implementation of this instance as well. Hell, either make it actually require 2 unique master accounts, or allow us to join it alone, anything is better than the current implementation.