This could work, however only for LEGAL factions only. Government factions are already shorthanded when it comes to their members and they need full dedication of the members in the faction, can't have it divided. As for illegal factions, joining multiple on one account is just more trouble than it is. CK clauses, possibility of shit tons of MG, etc etc.
There also must be rules like which factions u can join when beign in one atm. Some kind table. Ofc u cant be in different mafia tactions. And so on. Its a quite briliant idea.