If I knew BOTH clubs could stay in SL playing separately there's no way I'd entertain the idea of sharing.
However if your franchise was in danger, which if the RFL followed the letter of the law it probably will be should you have 0 movement on GH/WR (all hypothetical), I'd be more than happy for you to share either temporarily or on a full time basis.
The idea of Doncaster is laughable, WR to Keepmoat is 26.3 miles, WR to NM is 4.7 miles. That's before we mention that moving out of your city isn't seen in a positive light. We asked to play games at Oakwell (9.2 miles from BV) & were flatly told by the RFL that this was impractical. How many fans would go for that? What chance of catching passing fans? Even if you picked up some from Doncaster how many would travel when you eventually moved home? On an ethical level I would appose this, it would be a step too near the NFL for me, with franchises being picked up & moved at will.
Now I know the RFL are famous for changing their minds, but a move of the best part of 30 miles is stupid IMO, unless you change your name to Doncaster.
At the end of the day the season/my general interest in RL would be diminished by the loss of our derby. If sharing would save that I'm all for it, if we don't need to share EVEN BETTER!