Most likely you have two different cuttings that were grown out in the same pot. I got one on a load I brought in last week. It doesn't happen too often but it does happen. The only other and less likely way is if two varieties were an orange or yellow were grafted to the other color.
Only two ways I know of for this to happen.
Happy Growing David

Yep, I agree with David..this summer I saw quite a few bushy types of hibs that had two different colored flowers..
And like he said, it's possible they could have grafted two different hibs to rootstock, but this is less likely. I doubt they'd go through the trouble, unless it was specified, plus the price would be much higher than that of a non-grafted hibiscus..
Enjoy your bi-colored hib...Toni

Does one of the limbs come from very low on the plant? It may be a grafted plant and the rootstock ( a different color) may have sprouted a limb which is now blooming.