I would understand you can't install a NET3.5 build 6.3.10 and a NET3.5 build 6.3.12. But 6.4 is a totally new SDK with a lot of new features, nothing to do with the 6.3.x build.
In my opinion, Gizmox should allow to install both.
Now I will have to install build 6.3.13 when I am at work, and when I went home, unsintall 6.3.13 to install 6.4B1. So I will have to unstall/install build both a day. The problem with this is the license. I will not be able to use the designer because I will need each time to ask Gizmox to deallocate my license to able to reactivate it twice a day.
Please find a solution