To be clear… You can have more than one system on a Microsoft account but you need to be clear about which theme belongs to what.
For example, my work laptop (for technical reasons) has 8 & 8.1 on it as a dual boot… I use 8 at work for vmware and my desktop is the Chinese dragon theme that I downloaded from Microsoft but at home I generally use 8.1 with a custom space theme. The point is both systems belong to the same account i.e. I use the same e-mail, user name & password to sign in to them and they share all the online profile benefits | hassles.
Microsoft also allows what they now call a synchronised profile (in the old school language this is called a roaming account) & the idea here is that I can go to a friends' home and use her computer to sign into my Microsoft account… this gives me a limited profile and whichever background is set as default by me will always show up regardless of how many different computers I use on it.
p.s. A theme is a wallpaper background, sounds, icons etc