It's because they're the special ones that can alter reality if merged. First of all, that's why Hawk Moth is doing all his stuff to begin with, so the secrets have to stay safe for the same reason the kwamis can't know the recipes for the special element powers--if Cat Noir knows who Ladybug is and Hawk Moth gets Cat Noir...well, knowing Cat Noir, he'd die before betraying Ladybug, but we've already seen Cat Blanc, so. The point is, it's a liability. The other reason is that you never know when one of them might be tempted to use the reality-altering power for themself, and the best way to prevent that is to make sure they can't access the other Miraculous. Of course, if they're both tempted to use it and agree, then that measure is pointless, but you do what you can to minimize risk.
Of course, just because we can see where it does make sense after all doesn't mean we like it any better. Especially when they go and have another episode that tells us neither of them has had to give up their Miraculous years in the future. But surely they came up with a way to by-pass that rule, right?
I also like to think that Cat Blanc kind of proves the episode with that rule is just some parallel universe thing or something, because in Kwami Buster, Ladybug shows Cat Noir an illusion of Marinette de-transforming from being Multi-Mouse, so why would he even think that Marinette would be Ladybug? ...Well, because it makes more sense for them to have added that rule after the Cat Blanc fiasco, seeing as how nobody ever mentioned it before. Sigh. Oh well. I'm sure it will all turn out for the best.
As for the rest of the day, we wanted to spend it all playing Pokemon, but we had responsibilities to take care of. We're really wanting to just run away from it all, but I guess we're too realistic or something. Or we do the little things we know need to be done because they're a good excuse not to think about the big things that might need to be done. Times are tough, and we're not entirely sure the best way to go about improving them. For today, we just did the bare minimum of responsibility and endeavored to improve morale with Pokemon. I think it worked pretty well. We evolved Litwick and Carcoal! And we're starting to have a sizeable group of Pokemon at pretty high levels...I mean, for where we are in the game.
Today I'm thankful for errands and chores not taking all day, getting the "gourmet snacks" we ordered for our nephew's school fundraiser, getting to feel productive in Pokemon, getting to watch the Desperada episode of Miraculous again (but so many bad decisions these characters make...), and getting to sleep late again tomorrow.