You may have to wait till they are a bit older to start sexing, but I cut/paste this information I found online, below. Hope it helps. I've heard that male/female pairs can also not work out, that's why I have just one
The male can kill the female when she is not receptive to breeding, but yours are young. Give it time. They may be trying to establish territories. You may need to create some natural barriers, that block line of site, via drift wood, plants etc. You have a big enough tank to probably make this work, IMO.
One of the most sure-fire ways to sex bolivian rams is to examine the ventrical differences. In the picture below, we have a male in the top frame and a female in the bottom frame. Note that the male's breeding tube is smaller and more pointed, while the female's is rounded and a bit larger, since it's designed to pass eggs. You can also see that the male's body tapers downwards from the breeding tube to the anal fin, while the female's body tapers upwards slightly from the ovipositor to the anal fin.
Another common (though not 100% reliable) method of determining sex is by the dorsal fin. Note in the picture of the female above, there is a gradual slope to the rays of the fins, with only a very slight elongation of the third ray. Many males, on the other hand, have a much more elongated third/fourth dorsal ray, as seen here: