I'm not completely clear on the problem here but it sounds like you can't access some of the paged RAM area from one of the cores.
Firstly, this can definitely be done so it appears to be a software setup issue.
I'm not clear if it's the XGATE or the CPU that can't access the memory but each requires its own address to do this. In your case it appears that the CPU is using a 24-bit (far) access while the XGATE will be a 16-bit address & this may have something to do with the problem.
I suspect the best place to get an answer may be from the good folks at Cosmic themselves.