[support] 2 themes at once

[Roger]

  We have one theme, Marinelli working but it has no easy way of adding 
regions so I want to use mpFree which seems to provide all the 
attributes I need.
Question can I leave the marinelli theme working  and on line while 
developing the mpFree theme.

If so how can I switch between themes to see what needs adjustment.
Both would use the same database and content.

I'm stuck and would appreciate assistance
Thanks
Roger