There are a lot of things you don't mention. Some important things to know:
- Transmit frequency
- Receive frequency
- Type of cable between the radios and antennas
- How far each of these antennas are away from each other
- The condition of your antennas
But this is a good start at learning.
First things first: what you're doing requires not only a license, but frequency coordination. If you don't have either, stop and get them. Moving forward, I'll presume you have those.
Vertical spacing is worth more in terms of isolation than horizontal spacing. If you put your receiver antenna 50 feet in the air and had your transmitter antenna at 12 feet, you'd probably do well. That relatively little separation can be worth as much as 400 feet horizontally if the antennas were in the same plane with each other.
Your cable is extremely important. No RG-58 here! In fact, when you're dealing with full-duplex, you don't want any cable that has different metals or potential arc points (like LMR400). You need RG-142 or RG-214 at a minimum depending on how far you run between the antenna and radios. Heliax will do much, much better.
Your transmitter might be "noisy." Transmitters aren't always clean. Sometimes their energy is spread out of where it's intended. That will affect your receiving capability.
Receivers might be wide. They might not have the rejection abilities a "real repeater" would have, especially if they were designed cheaply and to loose specs.
If your antennas are old and corroded internally, you are asking for trouble. And even some new antennas don't duplex well. Hammie antennas that screw together corrode at the junction and create a place where mixing products and desensitization can happen. You need the right stuff.
Finally, frequency separation is critical. If your transmitter is 5 kHz away from your receiver, and your antennas are at the same site, it probably won't work. If it was 5 MHz away, maybe it will. If it was 15 MHz (not likely on VHF), there would be even more isolation.
The name of the game you're playing is called "isolation." Everything you do needs to contribute to the overall electrical separation between the transmitter and receiver.
Once you work your way through these things, you can start looking at duplexers.