On the contrary, you get better redundancy by sticking to one carrier and making sure that they really provide separacy though the entire span of the circuit. If you have two carriers running fibre to yoiur building down the same conduit, then you do NOT have separacy and as a result, the redundancy is not there.
The problem with this theory is that one carrier is completely free to reroute your connectivity among its resources.
One carrier can only do what your contract allows them to do. And negotiating a contract with strong requirements for separacy is easier with one carrier than two.
Note that many carriers, though perhaps not the LECs, will answer questions about the underlying resources they are using if they are sufficiently motivated, but you have to reask every now and again to make sure that the answers are still satisfactory.
Agreed. --Michael Dillon