Seems to me it sounds like you have two versions of operating systems in 1 tower and your trying to run both at the same time once youve cut the power on. Rule 1, never install multiple OS on 1 tower unless your running some type of network, not even then its still safe.
When you do that what you just did, it causes conflict in the start up BIOS meaning sometimes or most of the time or all the time, it dosent know which OS to load up.
Another thing i am thinking here is that you have two diffrent user name and password, but Im sure you have the same on both. If I were you, Id try to just reinstall 1 of those OS. The other OS can be used on another tower.
Im hoping all that was said by me made some type of sense or at least has something to do with this ding dang post. But then again, Im always sleepy when I type at this time of morning.