Unfortunately, you can not have two different versions of ArcPad installed at the same time. You will have to uninstall ArcPad 8 in order to install ArcPad 10.
You can, however, utilize checkouts from either version. If you check out with ArcPad 10 you should be able to consume that data with ArcPad 8 (and visa versa).
You'll have to install ArcPad 8 and 10 on different machines if you want to move forward with both versions. Only one version per mobile device as well.