Welcome to bunnies and BB
First, let me say, please keep them separated at all times. Bunnies don’t understand family units and being litter mates. When hormones kick in they can become very aggressive towards each other. Some bunnies on this forum have been injured when put together before being altered. Also, if you have a boy and a girl you risk pregnancies. Females can get pregnant as early as 12-13 weeks, which can be incredibly dangerous for them. Either way, they need to remain completely separated at all times until 1-2 months after they have been fixed, then they need to go through the proper bonding process.
To answer your question, litter training before they are fixed is difficult, especially for you having two bunnies. Unbonded bunnies will poop to mark their territory and it won’t go away until they are bonded. When hormones kick in they could very well stop peeing in their litter boxes as well. The best thing you can do is just set a foundation by wiping pee and putting it in the box, and placing poops in the box as well. Also, keeping their food and hay near the litter box because most bunnies will poop and eat at the same time. But as I said, the pooping won’t get better until they are a bonded pair.