Hmmmm odd one.
Have you checked the ATI settings from the advanced button? I don't know how much you've gone into this but you can alter the display settings from within these, but be careful, the advanced options relate to the monitor you have selected.
Select one and check the advanced settings for it, then select the other. It may also make a difference if the monitor is not active too so make sure they are both active first, even if they don't work.
Depending on the monitor, you may find you get more options with the primary than the secondary, in fact identical monitors will differ just because of the way graphics drivers work, but you may find something in there that doesn't seem right.
As far as I know, Max does not have anything relating to monitors within it so it can be narrowed down to a simple setting or a driver issue.
I've had problems with dual screens before but not like this. Sorry can't be of more help. I appreciate it can be a headache though.
Regards.