I consider that having cellphones on different networks is prudent in case of failure or congestion of the network.
After the terrorist attacks on the London underground, one cellphone network was unavailable, but the other worked fine.

In an urban area, the cellphone masts are closely spaced, and reliant on the utility power to the building on which they are mounted.
Localised blackouts can result in lack of a signal, a phone on a diferent net work may use a mast on a different building that still has power.

It also protects against loss, theft or breakage of one phone.
I have two cellphones of my own, and a company one. I would not normally use the company phone for my own calls, but would in an emergency.

One network at present is giving no coverage at my home address, presumably due the great power failure at present.
Its been off for a couple of days now.