Cable as a backup for T1?

I work for a small company that has a T1 coming into our main site. 6 remote sites are also coming into the 2 servers through a VPN tunnel. Most of the remote sites use cable at their internet line. The main site has an opportunity to consume a cable line here to use as a backup for the T1.
Does this make sense? Will it crash (about 25 users coming in on VPN tunnels connecting to Terminal Services)?
How about as a an additional line? Can this work?

Thank you,

Ron