The two account rule makes sense especially given that in most casinos you can register as a referral and the person who referred you keeps getting rewards based on how much you gambled. So especially for platforms that give a cut out of everything wagered or lost, this effectively means lower house edge advantage for them. With multi accounting this can be used against the casino, even more so with low house edge games it's an excessive risks.

I would say though it's kinda hypocritical that casinos enforce this rule sometimes to withhold big wins. At least they should close the account and teach a settlement with the punter to find an acceptable solution for both parties.