Well, what I would do is, un-mount all virtual drives and the set the number of virtual drives to just one, if you can.
Then browse to your .iso file for Disk 1 and right-click on it.
You should see an option that lets you mount the image to a virtual drive.
Mount the image and box should pop up.
Pick 'Open folder to view files', find the setup program, right-click on it and 'Run as administrator'.
When it is time to insert disk 2, browse to the correct .iso image and mount it onto the same drive as the first one was.
You may even have to un-mount it first.
The installer should do the rest.
If you need to change disk again, do the same thing, mount the new disk on the same virtual drive as the old one is on.