Running multiple T1 lines isn’t normally a problem. Your ISP should know how to aggregate them into a single virtual connection. We just moved up to our fourth T1 connected to a single router.
Another possibility is to use a partial T3. With this arrangement you have a T3 to the ISP but only contract to use and pay for part of the capacity. If you anticipate significant changes in connectivity needs this may be a better option. When you need more bandwidth you just call the ISP, they turn up the bandwidth, and you get a bigger bill the next month.
You might also want to look at other options like DSL. You could connect the sites using a VPN.
As you can see, there are a number of possibilities. You should weigh each one considering price, availability, functionality, and security and decide which is best for your environment.