There is an imbalance there. Our party seems to have solved it by an unspoken "non-aggression" pact, where once someone has Cleric spell casting nobody else tries to take it because that would be stepping on their toes.
Cleric and Wizard spellcasting and Druid shapeshifting are three cases where the heart of one class can be copied by a single multi class move taken by another. You don't have to be ruthless in your min-maxing to notice that those are powerful moves.
There are a few ways to rein this in if your players aren't the kind to do it on their own. One is to fall back on the "to do it, you have to do it" rule -- make the players explain how they acquire something as amazing as the favour of a god, when they didn't have it last week. The Cleric probably was born with it, or at least spent years as an acolyte. What did the Paladin do, save that villager? The whole party did that, why are you so special? Play it out. It could be awesome, and then it's all worth it.
Or else you can houserule in any number of ways, such as requiring more than one advanced move slot, or giving a penalty to spell casting if taken multiclass, or whatever. But all my ideas here are pretty klugey, and I expect that's why they're not in the rules.
Dungeon World is not a wargame, and has serious flaws if you play it that way (what's the range on a Fireball, anyway? What's up with Cloudkill? Or Soul Gem? That's a weird spell.) But it mostly works out if you focus on making the rules serve the story. Either the broken thing makes no sense in the story, and therefore you can't do it, or else it pushes you to get creative about explaining it, and that makes it cool.