IS boxer and Keyclone both allow for this, But without using software, the easiest way To me, would be this.

If you have multiple HD's or particians, just load one game from each.

For example, WoW1 loads from the C: Drive, and WoW2 loads from the D: drive.

You can set each up then to what monitor you want it on, and go from there.
Sorry, just finished reading you didn't want to double your HD Foot print. The best thing to do then would be via MB software.

With Keyclone it would be as easy as setting it up n maximizer. With ISboxer, (going with a very poor memory here) , It would be through the monitor on the lower menu. I won't swear to that though.

<sigh, I really need to buy some more time with it, as I like ISboxer also.>

Stephen