Now, true if she did, there would then be two twilights. But one would be an alicorn and one a unicorn. Reason, The human one never learned about friendship from what I believe. And it was do to Twilight's time with her friends that made her turn into an alicorn. So do to the human twilight not knowing friendship, she wouldn't have the power of friendship. Keeping her in a unicorn state.
What balance would this effect? Maybe nothing, maybe something. Because there's no real way to know if the human would even had a sunset shimmer to beging with. But if there was, then the human twilight going over could even it out. Two sunsets in the human would, and two twilights in the pony world. But if there really is only one sunset, then that means if the human twilight finds out and goes over. The balnace would be off. True when pony twilight when over, there were two in that world. But it wasn't ever for long. Just a few days.
WHich shows a bit more of there only being one Sunset shimmers. Either nothing really does happen when two are in the same world for a long time. Or there is just the one. And even if the human twilight came over. It wouldn't effect the elements. Becaus ethe sunset we know is now the element of friendship, where the other isn't. And for the twilight's. One is the element of magic, and the other isn't. Again, balance.