Use 2 T1’s on same network

I have 2 T1 lines from different ISP’s, 1 network/domain, SBS server. The T1’s obviously have different gateway’s. One line is not used often, mainly for backup purposes.

Each T1 has a separate router,firewall and HP switches. Only 1 (ISP A) is connected to the domain/network.

Can I have both T1’s connected to my network/domain thus allowing me to switch over management and some employees to less used T1 (ISP B) without them losing access to network resources (drives,SBS,etc..)??

We have several PC’s with 2 NIC cards, is there an alternative to purchasing additional NIC’s and running more CAT5 cables?

Thanks in advance!
Jason