You should each speak your own language to your children. You'll have the best success if you're very strict with yourselves about this--you have to speak 100% in your native language to your child for their entire childhood. Make sure your children respond to you in the appropriate language as well (although it's normal for them to mix languages for a bit when they're only a few years old, and that isn't necessarily a sign of deficiency in either language).

Since your kids are only learning two languages, I'm assuming either you or your fiancee are living in your native country. Your children will learn the language of the country they're raised in with zero problems. Even if nobody spoke that language to them at home, it would still happen naturally as they started going to school and making friends. That language is also inevitably going to be the kids' dominant language.

So the parent that speaks the foreign language is the one that needs to work hard in this case. Kids need a lot of exposure to it: the parent, relatives (if possible), visits to the foreign country (if possible), books and TV, schooling (if possible), etc. It also needs to be maintained for the kids' entire life--if you give up when the kid is, say, 5 years old and the parent starts gradually speaking to the kid in the other language... The child could lose their ability to speak the foreign language, even though they were able to speak it at 5 years old. Language is something that needs to be maintained all the way through adulthood.

If the partner speaks the foreign language (even though it's non-natively), then you might even consider both of you speaking only the foreign language at home. The child is already guaranteed to learn the home country language, as I already said. Whatever you do, just make sure you get your kids as much exposure as possible to the foreign language. It can take some effort, but native bilingualism is something your kids will probably be thankful for when they're adults.