It is now possible to assign templates to multiple designs. It was released about a few months ago if I remember right. The thread should have been updated with the new change.
To assign the template to more than one design, you just need to multi-select any pages you want the template on. This will make all of the designs for those pages appear in the Design Pane below the Page Tree.
After the designs load in the pane, select the designs you wish the template to be on and right-click any of them. There will be an option to Edit Template.