Wow. Did you mean S12D64 vs S12XD128 or S12XD64 vs S12XD128? I thought you were talkig about S12D64 vs S12D128.
S12D64 vs S12XD128. Too big differences. I would not bother trying to do binary that works on both. Something is possible, but not easy and won't work with all tools.
S12XD64 vs S12XD128. That's easier than S12D64 vs S12D128. Just compile for smaller XD64.