you should be able to split the 24 lines (making it 64k intervals) any way you please. this is of course providing your telco lets you. you can, depending again on telco support, get a blockl of extension numbers for the t1 line. using multiple t1s should not be a problem providing you have enough wires into the building from the telco and you have the appropriate hardware to loadbalance onto them.
as for multiple files at t1 speed, can you start 10 files on your 3mbit cable line and receive each one at 3mbit? that should make the naswer obvious.