Each property can only have a single value (otherwise, how would MODX know which value to use to replace the placeholder tag).
If you can give more details about exactly what you're trying to do and why, we might be able to suggest another way to do it.
There are lots of ways to go on this. If each template has a constant set of JS includes, you can just put the lines to include the JS in the template itself. That's certainly the simplest solution.
You could also put different sets of JS include lines in different chunks and make the selection by which chunk you insert.
If you need to select JS includes on a page-by-page basis, with different sets used for the same template on different pages, it gets more complicated, but let's not go there unless you really need that capability.