I have a DSL modem-router . I have another modem, can I hook that to the same phone line

Not possible.
It would be the same as connecting two phones to ONE connection and then using them to call different numbers.
You need to share the connection that is coming in.

Actually it depends on how you define "phone line". Most conventional phone lines are made up of 4 wires. But as far as DSL goes or your phone it only requires 1 pair (2-wires). So in if you activate a second line then you can have a separate modem attatched to that line. 1 pair per dsl line.

Yes, you can plug router 2 into 1 of the ports of the first router, and if both are wireless routers, the second will act a WAP.
Can you be more specific what you want to do?