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Adding female was method hoped to keep male's interest inside fenced area. Periodically confining one or other will hopefully keep them inside.
are they both going to be spayed/neutered? If not, then you are going to have 50X the problem when she comes into heat. Male dogs will pack up on your male to get to the female. If your dog is neutered and she isn't, then she could likely join in the attack on your dog if he tries to chase off the intruder.
You said that the terrain is too rough to be able to put in a fence. How are you going to mark the boundary for the dogs? They need a clear and precise border for the training to be effective.
I used to use an IF. One dog it worked on, the other not so much. He would actually stand at the inner boundary of the fence, studying, then charge through. He yelped loudly when he got the shock, stop on the outside of the boundary (after they get X feet on the other side it won't shock again) and shake it off, then continue on his merry way. When he was finished with his wandering, he would sit outside the outer limit of the fence and wait for someone to come and get him. Only a couple of times did he brave the shock to come back in alone.
Besides that, here are the other problems I had with the fence:
burying the wire. OMG it was a freakin' nightmare!! You can't have the line too deep or you risk losing signal strength. You can't have the line too shallow or you increase the risk of breakage from heat/cold expansions. There can't be any rocks or roots in the trench or you risk wire breakage.
Checking the wire. Not too bad when you have a small, clearly defined area. Just have to do it on a regular basis, testers are fairly cheap. When the weather is bad, you have to check it more frequently. Even the tiniest break anywhere and the whole system is down.
Batteries in the collar. Those things aren't cheap. And if your dog has a tendency to approach the fence and trigger the warning beep, the batteries will wear out faster.
The collar needs to be checked daily for snugness. If you have an active dog that is running through brushy areas, I'd check it more frequently. If the collar is too loose, then the collar won't make contact at all or at least at a reduced strength. Collar too tight and you are going to create sores on the dogs neck.
Weather. If it rains, you are going to have to take the collar off and allow the skin underneath to dry completely. Otherwise, you run an even greater risk of sores on the neck.