While I haven't tried steam's version, I would imagine that you're still required to register your own e-mail/password to run it.

So without buying a second account, you should be able to test it with ISBoxer or whatever else you use. Simply find where the executable resides, and set it up in a similiar manner. If you're able to run two copies, then you'll be set.

If steam has some limitations, I would try downloading it directly and running it through the launch with the same e-mail/password you used when you registered it.