I'm a bit confused by your question. You referred to placing another block with mates, but you only included a single part, not an assembly, so I'm not sure what you're trying to do. If you only want a second body in the part, with a 5mm gap between them, one option would be to create a plane 2.5mm from one surface, and use this plane to mirror the body. That's what I've done in the screenshot below.
If you did mean to create an assembly using two instances of this part, then mating the second to the first with a distance mate shouldn't be that difficult. You said you're fairly new to SolidWorks. Have you worked through some of the tutorials that are included with the software?
Thanks for the reply Glenn, I managed to figure it out with a little help from your good self!