can anyone answer this

Well only one can work as modem. The other one is probably a router. But you would need only one of that as well, so 2 devices max. I would prefer a modem-router 2 in 1.

You connect your cable box to a modem. You can then connect the modem to a router and you have Wi-Fi for all your devices. You only need one of each, 2 devices total. So it would appear you have one newer modem, one router and one old modem which was probably to old to handle newer bandwidth needs.