How to combine two small bathrooms into a larger one?

Main thing to sonsider is if the wall between is a load bearing wall. Which is unlikely. If any of the fixtures are are against the wall it may be a "chase" for plumbing, electrical, and HVAC runs. Back to back bathrooms are usually built that way. Either view it from the attic if you have one or cut 4" inspection holes and use a mirror and flashlight to look inside. Holes are easily patched.

Separate toilet room is a must! Pocket door saves space.

Hi Joyce. Congrats on the new house. Since your plumbing will be in the common wall, you will need to consider how to move all that. There are code rules about distances beside and in front of sink and toilets. While you're in that process, consider either making the sink(s) and toilet separate from your bath and shower or ekeing out a small area for a half bath. You'll be glad you did. Oh, and don't forget your permits. Big Help for Small Bathrooms | This Old House

From a perspective of the value of the home, I would discourage you from entertaining any schemes that reduce the WC/toilet and sink areas.

Moving any plumbing can be very expensive.(read: thousands) As William noted, often HVAC, and electrical need to be moved as well, which further adds to the cost. Consider moreover perhaps moving/shifting doorways, installing pocket doors, and creating a Jack'n'Jill type floor plan. What rooms connect on the either bathroom? Have you looked into the possibility of converting one of these rooms into a dressing room/bathroom combo room with vanity and sinks?