What will happen if I take two zolpidem 10mg?

20mg of zolpidem is almost double the standard high dose of 12mg, I would not recommend taking 20mg, although I doubt that you’ll have too many I’ll effects.
You will likely feel groggy and somewhat in a haze the following day, Zolpidem should not be taken without caution.
Also it is worth noting that Zolpidem is typically prescribed as a short term solution to insomnia and does not induce natural sleep. Be safe and stay on the lowest dose you are prescribed, especially since Zolpidem can make you unaware of your actions following administration.