When putting lovebirds together, you want to let them get to know each other first, so put their cages close so they get used to each other. Then you can let them out of the cage together for short neutral supervised visits. You'll get to see how they interact at this point. Sometimes, it's love at first, sometimes not. After a few times of that, you can give short supervised cage visits. You should see by now if they like each other or not if you've done this a few times. You will want to be sure you remove anything even remotely nest-like from the cage first and also any mirrors or 'buddies'. In the case you have unknowingly put together 2 females, you want to avoid anything that will produce strife.
I want to add that unless these birds were DNA'ed, the only way to truly know the sex is to know one laid an egg...and just because one didn't does not make it male. This means you have to take care when putting them together in a cage because females can and will be vicious.
You should expect to see some squabbling though, they will test each other and that's part of the process, that's why it needs to be supervised, there is a pretty fine line between normal squabbling and tearing the other's throat out