Bathrooms with two doors aren't popular. If you were meaning the ability to close off one door and open another permanently, that could leave one bedroom without a WC on the same floor, which would not be popular. It might also not comply with Building Regs, which would have to be applied for when you change the layout.
A bathroom on the top floor with only one bedroom there is fairly private and will suffice in lieu of an ensuite, but provides the flexibility of being used separately if needed (per Sarah Beeny).
Depends on your target market, the locality, and the balance of the house - eg potential buyers for a 4-bed house are more likely to expect
2 separate reception rooms.
If you plan the strengthening beams properly you will probably have spavce to run soil pipe under the new loft floor. Otherwise there's always s*n*f*o.
Keeping the available headroom for the habitable rooms is definately best. You ned a bit of headroom in a bathroom if you want to get a shower in.
Owain