|so you confirm that i can just plug a 2nd router with its own IPand all to the same switch the 1st router is connecting to?|
Yes, I can confirm that. We have two external connections, from completely different providers, here where I work and they both connect to the same switch.
Keep in mind that there is still some setup required to make it all work. You won't just be able to plug the new T1 in to your switch and expect it to work. There are changes you will have to make. If you're not sure which changes you need to hire a professional consultant to help you do the changeover.
which dns will the domain controller be pointing to?
That depends on which one it's using now. If you have your DNS server forwarded to your ISP's DNS server then you will need to change that to the DNS server(s) of your new provider once the new T1 is connected and working.
Again, I can't stress enough that you need to leave the old, working, T1 connected until you're 100% sure the new one is working.