There's no problem w/ practicing BMSG and XWXG at the same time. In the novel there's a part where TSTL got mad when she noticed that besides BM zhenqi XZ's internal energy also contained XWXG energy (LQS taught this technique to WYZ). Not sure about the I Reign Supreme Everywhere technique, but b/c TSTL could teach XZ BMSG and knew a lot about the properties of this technique I don't think she didn't master it. I need to check the novel again later.
TSTL was mad cos XWXG was taught by their teacher to LQS and not WYZ. The fact that XZ also had XWXG meant that they exchanged skills and to her it meant very close relationship. It seems to indicate that they each learn a separate skill and did not know what the other knew. Of course, this contradicts the fact that she actually knew BMSG when she taught XZ. Maybe she knew the skill but was not supposed to (or cannot by reason of either teacher's instruction or more likely because she was already learning the Everywhere skill) practice it.

As for learning mutiple skills, there isn't a problem, IF the first skill was BMSG. The first thing to do for BMSG is to destroy everything else. TSTL started learning her weird skill at age 6, so the only way for her to also have mastered BMSG is for her to learn that at age <6 or that she had destroyed her own skills at a later date, one being ridiculous, the other being stupid.

P.S. I have not read the third edition, but what I heard is that her weird skill had a new name. Did that affect her age reversal in anyway? Did anyone list out the changes and, if so, can someone point me to it? I tried the sticky by Athena, but I couldn't find DGSD.