Is the ONLY second one (48V) enough to provide router+PoE supply or I need both connected anyway ??
Your device can provide only input voltage(s) on PoE-out. When both power supplies are connected, then CRS can select (or you can set manually) to provide either low- or high-voltage on each PoE-out port separately. Your CRS doesn't do voltage regulation for PoE-out, so if you have some passive PoE client that requires low-voltage, it'll be cut off when you disconnect the low-voltage power supply. If you have none PoE clients that would require low-voltage, then you can safely disconnect the corresponding power supply. The same is true if all PoE clients were low-voltage ones, in this case you could disconnect high-voltage power supply.
Note that router itself can run on either power supply and will down-convert supplied voltage to levels actually needed (probably that woukd be 3.3V and 5V, possibly also some lower voltages). If both power supplies are connected, then router will[*] use the higher voltage source for own consumption. Both power sources will then act as redundant power supplies of the router itself (redundant against failures of power adapters, the rest of redundancy depends on how those power adapters are connected to AC power supply).
[*]This at least happens with other MT devices which accept multiple power sources, they always draw pover from highest-voltage source available at any time. If highest-voltage source goes AWOL, then device starts to draw power from the remaining source providing then highest voltage.