My H and I are informally sharing our dog. I had him for the first 6 months of our S, except when I was traveling. H had him while I was gone for the summer. I have him again, but H will when I leave town next week and for Xmas/NYs. Each time we exchange him, there are a few days to a week of adjustment to the new routine, but then he settles down.
If you really want to share your dog -- and in spite of the surprised responses of some here, I know several estranged couples that do so -- I suggest a two-week or monthly schedule. Just keep his food and routine and discipline as consistent as possible (like kids!) and he should be just fine.
M 65 H 64 T 39 & M 36 @ S 12/08 Two Ds
Do you know that the harder thing to do and the right thing to do are usually the same thing? Nothing that has meaning is easy. ~ The Weather Man
