Using two templates in one presentation is not possible.
However, you can copy any element or even an entire template from one presentation to another. To do so:
Open the two presentations you would like to merge in “Edit” mode.
Select All (CTRL/CMD+A) in one of the presentations and either
- copy and paste the contents into the other presentation.
- add the selected content to Favorites by right-clicking and selecting Favorites. Then in the other presentation, insert the content by going to “Insert” and clicking Favorite. If you cannot see your contents in the Favorites sidebar, please refresh the page.