I find it’s always easier to just start modeling instead of wondering too much about what would be the perfect approach. Afterwards you might find the better way, and then do it again:-)
I would suggest using dynamic components from the 3D Warehouse. Just put them next to each other and I wouldn’t worry about the stool.
The Marvin Double Hung window can be changed to whatever dimensions and number of panes you need. Just change the options after inserting it into the model.
or
There are other dynamic window components in the warehouse as well.
I use Marvin windows a lot in my projects, and the dynamic components in 3D Warehouse have been very helpful many times. The top example in @DanRathbun’s post is often called a “cottage window”, and I think your photo is then sometimes referred to as a “reverse cottage.” Such names can be very regional, however.