If you are going to use both at once, the instructions on my Syrabicort recommend that you use the Xopenex first, so that the other medicine can penetrate deeper into your lungs.
So yes, it is allowed.
Fair warning, Syrabicort makes me a little wired, and Albuterol (similar to Xopenex) makes me a little wired, and the first time I took them at once, I was VERY wired for a while. (Now that I'm used to both drugs, it's ok.)