Check http://www.nvars.com/
In Beijing, the H1B visa is only single entry in three months as listed on the website of U.S. Embassy in Beijing. I am afraid you won't get 2 entries from Canada. (You are the same person no matter where you apply for it.) Why not apply it in China directly?
I am going to apply in Beijing directly for H1B visa this winter. At the same time I will help my mom to get her visitor's visa (B2 I think). I hope everything will be smoothly done.