Can you have 2 wireless routers at the same time into 1 modem?

Yes and no. The no is that a modem can only have one device plugged into it, a router of some sort.

The yes being that you could have multiple wireless routers, acting as access points, on one network. As long as they have unique internal IP addresses and are broadcasting different SSIDs on different channels (unless you are bridging, which is way beyond this) then you could have different wireless networks on the same connection.