Have you dug down into the soil to look for a graft union on the main stem? There's wouldn't be anything intrinsically wrong
about growing an ungrafted hibiscus ... that I can think of, anyway. Regardless of whether it is truly an entirely separate plant, or a sprout from the rootstock of the other plant, I would remove it and put it in a separate container. That way, if the secondary plant turns out to not be something you want to keep ... if the quality of the flowers is so poor to that it isn't worth the effort and expense of caring for the plant, for instance ... then you can dispose of it easily. If you allow it to grow in the same container as the original plant, the roots will become entwined, and trying to separate them later on may damage the plant you want to keep. Also to be considered is that, living as far north as you do, if one plant failed for some reason, being in a separate container might allow the other to continue to survive.
Placing the new plant in its own container will also ensure that each will have sufficient nutrients and water available to it.
If, when you dig down to the roots of the secondary plant, you find that it is attached to the roots of the original plant, just use a clean, sharp blade to sever that section of the root(s) from the main root ball. Make sure you take a reasonable portion of the main roots, so that the plant can continue to live on its own.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams