The default assumption when talking about two people is "to each other". There are no other people that you might be talking about, and the default assumption is definitely not "to other people they haven't met yet."
You'd have to say "They will get married, but probably to other people" if that's what you meant.
If the two people in question are not even potential partners for each other (e.g. close relatives, or known incompatible sexual preferences (gay/straight)), you just wouldn't phrase it this way. Even if your audience also knows that they're related, or highly unlikely to be interested in each other that way, the implication of "to each other" would still be there. So it would sound weird.
If you don't mean "to each other", you could say "they'll probably both get married", because that phrasing implies it would be possible for one to get married without the other. That would be the implication for this phrasing even for two people that are potentially romantically compatible (e.g. two gay males, or a straight guy and straight girl).
You could shorten even further to "I think they'll marry after they defeat Dracula." However, I think that fits better for people that are already a couple. It makes me think you're taking the getting married as a given, and you're guessing about the time it will happen.
For people that haven't shown any obvious romantic feelings for each other, it seems to fit better to say "I think they'll get married after they defeat Dracula". That feels like you're guessing about them hooking up at all, and the specified time frame is secondary.
I'm a native English speaker, born and raised in Canada. It's possible that other English speakers might have a different understanding of this, but I'd be quite surprised. Especially the "they'll marry" phrasing makes it clear that we're talking about "to each other". That implies it's something they'll do together, and "to each other" is what normally happens when two people get married as a joint activity.