Whichever SIM you choose as active will be 4g (if your phone supports it, or 3G, since you have not mentioned which phone you have), the other one would be limited to 2G
I don't think there is any device that can use 2 internet connections at the same time - it's not done at all
If you need additional help or have more questions or details to share, please join the site so you can reply in this thread. See this link for instructions on how to join Windows Central.