MickE,
This is a little quarky, but try the following and see if this gets you what
you are looking for:
Open project A (will end up on the right side), then open a new project file
(File | New | blank project, etc...), then open project B. Select menu option
Window | Arrange All, then select the "new" file and close it. You should be
left with the 2 files side-by-side that you wanted, and will only have to
resize the one on the right to be full hight. Note that if you use the Window
| Arange All again you will rearrange the windows on top of each other
instead of side-by-side.
I hope this helps

!