" a tub and shower in the master ensuite "

Separate tub and shower in the master? Too tight for both. In fact both of your new bathrooms would be tiny and probably not really useful.

What about the idea of making that bath exclusively for the master, and putting a second bathroom in where the current walk in closet and closet are, opening into the hallway. You could add new closets to your master.

Could the flex area become the guest room, and the guest room, become another bathroom. I wouldnt want two tiny bathrooms and thats what you would have by splitting the one you have now.

Provide the guest BATH robes: ) or its a child's bath and it doesn't matter.

Here is the math: splitting horizontally won't work, as each bath would be less than 4' 6", after allowing for dividing wall thickness.

Splitting vertically could yield a 5' x 9' 5" guest bath, leaving 5' 10" x 9' 5" for the master bath. That is an adequate guest bath and small master bath with a tub/shower combo and single vanity.

You might be able to do a staggered wall, but you'd be giving up more floor space for the dividing wall. Moving all that plumbing won't be cheap. Having a second bath is certainly desirable. A local real estate agent could offer advice on the desirability of a small master bath in your area.

That size, split in two of anything, will not work as each must be accessed individually, each must have toilet, and bathing. The window would need to be eliminated and Both will be too small to be useful. Steal from the flex, or forget it.

I think the issue is access to each once the space is split.

I wonder if you made the flex space into the bedroom and the bedroom into the flex space it would work better. You would have to remove and juggle walls

If it were me, I would move the bathroom doorway to the hallway only, making one bathroom accessible from the hallway. Then I would have room for a double vanity. I might also consider using some of the flex space to enlarge the master closet.

We had this issue in a previous house. We built a luxurious master bath but it was not en suite. We had three additional bedrooms on that floor and only the one bathroom. The additional bedrooms were only used ocassionally for out of town visitors. Most of the time, it was just the two of us.

I juggled the bedroom into the flex space. Some houses here have this situation where they have windows that open onto the open double height area you have at the front. But you would have to have an egress window somewhere. The flex space could be made a little smaller, the bathroom complex a little bigger, doorways could be positioned various ways.