i had something similar once.
For me the solution was to make sure you had two wow directories.
One for slave and one for master. Then first launch the main account, set it to windowed mode, and make sure it has the correct aspect ratio,
Then load the slave copy from the other wow directory, and make sure that one aswell have the correct windowed mode with correct aspect ratio.

With aspect ratio i here mean that it should have the same aspect ratio as the windows you setup with maximizer.

For me i set all windows to start in 1440x900 as this was the aspect ratio of the main window. Each of the other windows run same aspect ratio
but loads as 1440x900, then maximizer set them to 720x450. Setting the windows to have same aspect ratio also made them load faster from what i could see.

Something to try atleast.