Speaker cables for mid and high

I am running bi-wire with Analysis Plus 9 for low and Analysis Plus 12 for high. Should I have to use the same gauge for both?
No. Smaller is fine for mid and tweeter. The larger gage is useful for bass but may be worse elsewhere. But as always let your ears be your guide.
Agree with Stanwal 100%.
Thanks. One more question, can I use different brands of cables for low and high?
That is an experiment for you to try.It is possible,but there can be imaging differences among different cable brands.
I have done it, most recently using Mapleshade for top and Anti Cable for bottom. Inconclusive results. As Tpsonic says results will vary as every brand will l have its own electrical characteristics. There is a very interesting debate now going on on the HIFICRITIC forum site about the value of biwireing at all. As for myself I have stopped doing it, partly because of cost , but also not sure it is always better.