Definitely post a picture of the two side by side 40" windows. If they're close enough together, I bet you could do two panels with an opening in the middle.

I have the shutters in two rooms, and valances, tiers in the other rooms. I like mixing it up as long as I don't have both in one room.

They are about 30" apart. I've been looking into different mounting options, and I'll have to re-measure for an inside mount. It may indeed barely fit with only two panels, mounted that way.

Do you need to be able to open the shutters? I have 4 1\2 inch shutters and when I just open the slats it lets plenty of light through. I never actually open the shutter itself. I will try to attach a picture......

Love the table! Thanks for the pic. We would like to open the shutters sometimes. Won't leave any open, though.

Here's a pic of the smaller window:
Best I can tell the mounting spot would have to be removing the 3/4 x 1/2 ogee molding and installing a mounting strip there. Only other option would be a full replacement of the casing with a trim (that could direct mount the hinges).

there are so many windows, I don't want to replace the entire sill and all the trim, if I can avoid it.

Here's the two taller windows together. Pardon the curtain mess, we were trying to see if we liked curtains that only came down to the sill (instead of all the way to the floor).

We would remove the bookcase (eventually), so being able to open without collision would be important.

There's a slim chance a 2 panel would fit if inside mounted. I'll have to confirm the measurements.

Hope this help give you an idea of what yours might look like....here's ours. We have three of the same size windows in our living room.