Howdy cp1 and welcome to the forums!

you should be able to type the resolution you want directly into the resolution box.
try 1680x700 as the resolution for one, and 1680x350 for the other.
to do this, just type the resolution directly.. don't bother looking for it on the pull down

if that does not work, try setting them both to 1440x900 .. a matching res of 1.6 (1440/900)


also.. keep in mind, if you change the resolution, you will need to restart wow for it to take effect