The simplest thing to do is to activate SFH 1×1 on all the borders sites. The frequency planning of other sites might be a little impacted (I mean the sites which are a little inside the country already, not at the border)
The advantage is that your site is virtually interference free, or at least as much as it can possibly be.
For even better performance, try to reduce the number of TRX per cell and activate AMR HR.
The other solution is to perform regular drive tests in this area (or contact the foreign operator) and find free frequencies. But this is very tedious, because the foreign operator can change frequencies anytime, and everything is to be done again.
Cheers,
pix