Yes, this is a bit of an odd one since that size is pretty far outside what is listed. Ultimately, you need the width or depth to be significantly larger than the side return dimension. For example, you wouldn’t want to order a Wall 90 cabinet at 26″ wide and request a 24″ right-side return, since that would leave barely any space for the doors.
As long as the dimensions are within reason, though, you can simply order the Wall 90 cabinet at, say, 36″ wide x 36″ deep with a 12″ left-side return. If you want the right-side return to be 24″, select the largest available right-side return from the dropdown when ordering. Then, on the checkout screen, add an order note requesting that the right-side return be changed to 24″.
For example:
“On the W90 cabinet ordered at 36″ wide x 36″ deep, please change the right-side return to 24″. Leave the left-side return at 12″ as ordered.”