I'm new to the field and still learning.
In my business, we are NOT 100% dependent on telephone service. It's nice to have 100% up-time but we can suffer some down time. If your business is dependent on 100% up-time then, a second T-1 might be worth the additional cost. If your business is like mine, I would think that a second T-1 would be too costly as a backup for just 70 people. If your business "stops" when the phones go down then a second T-1 might be worth-while however...
I've found that the major sources of my problems has been the "local loop" not the carriers. Typically, at most sites, the T-1 and the POTS lines are contained in the same cable in the street. One dig-in or flood would wipe out all your copper so, the problem would be with the local loop and not the carrier. I mention flood but this covers a lot of ground (pun intended). We suffered problems a few local loops for about a year. The wires to our site from the CO came through a pedestal located about a block away....right next to a lawn sprinkler. Sure enough, the cabinet wasn't totally water proof and the copper would get wet. It took the phone company about a year to figure out what was going wrong. Since all the cables came through the same pedestal, a set of backup lines wouldn't have helped.
mike