Sorry you're having to go through this....
It's impossble to generalise but I was in potentially a similar situation with losing Twilight earlier in the week. He and Mulberry were the closest of all my "couples" yet when he started to weaken she simply transferred her affections to Merlin, started ignoring poor Twilight, and has been happily snuggled up to Merlin ever since. :shock:
Sometimes rabbits "know", and I guess, like humans, they deal with loss, and the prospect of loss, in very diverse ways. It's impossible to predict how things will pan out in your situation, but just wanted to post to say that it may be a lot easier than you think. Mulberry is still showing no signs of being bereft at all.
The fact that they're together may mean that the deterioration is being sensed, and transferral of affection will be easier, as it has been with Mulberry. Or it may not - just wanted to share what I've witnessed this week, as it's really surprised me....
Really sorry though - it's a heart-breaking situation to be in....