Remove the curtains and install Roman blinds. Make the top of the blind on the left the same height as the higher window on the right. Open both of the to the same level daily.

Or faux wood blinds, hung the same height - the curtains could stay or go.

Maybe some plants under one of the windows to disguise the different heights at the bottom. Maybe once you address the tops of the windows, the bottom wouldn't really be noticeable unless you knew they were different (no one else would probably notice unless you pointed it out)

The dark drapes (dragging sloppily on the floor -- pet peeve of mine) actually emphasize the difference so, even if you keep them on one window, don't keep them on both (and hang them higher).

The most obvious solution, budget permitting, is to swap out the single window for a French door. Then you get to create a small (private for this room) patio.

Alternately, you could do something to make the windows have a visually different purpose AND then it would be logical to use entirely different window coverings.

If you want to visually change the single window, you could panel the wall all around it using wood boards with a wood finish -- not painted -- and then use wood (or faux wood) vertical window blinds with the same wood finish that fit within the window, not extend above or beside or below it. (You could do the same for the double window.)

If you're changing the single window, you could even create a decorative feature in the larger space above the window.

If you don't want to add paneling, you could create an accent wall with paint, perhaps including adding a vertical board between the windows from baseboard to ceiling and paint the wall two starkly different shades of the same colors -- different on each side of the room -- then set a wood bench beneath the double window (with space beneath it so that it doesn't interfere with any baseboard heating), painting the bench the same color as that side of the wall. Then add vinyl vertical blinds the same color as the painted wall around the double windows to cover the windows only -- stopping the length of the blinds at the windowsill.