One work around is to put your secondary monitor on the right of your primary monitor. This will make the theme display correctly on the primary monitor. Your still going to have a problem getting the theme onto the secondary monitor.

Also to improve themes compatiability with dual-monitors I would would used jdesk and rabidvwm instead of desktop2 and ckvwm. Simplicity uses these last two modules and it shouldn't be too much effort to change them over. In most cases the variable names remane the same.

The reason why the themes get split up is because some modules interpret the co-ordinates 0,0 as the top left most corner of your desktop whilst other interept it as the top left most corner of your primary monitor. You might be able to get some elements of a theme onto the second monitor by using negative values.