In Power gallery 2, on the chose a design page, you can select the gallery design to use, you can also edit the settings to create a new preset with a new name.
For the gallery you wish to use, select the design, then click the gallery to set it as the selected gallery. go to the add to page tab to get the code to add the gallery to the page with the selected Design theme.
Go back to the Design Tag and chose the other design theme you wish to use, then click the design again to set it as the active one. On the add to page tab, then new code will be connected to the selected design theme.