Transmission line, cable impedance
You can get the effect of a 50\$\Omega\$ impedance line by putting two 100\$\Omega\$ lines in parallel, if you have sufficient number of pairs to do that.

The reason differential lines tend to be 100\$\Omega\$ is they expect the user to use an anti-phase pair of 50\$\Omega\$ drivers and receivers for them. Being anti-phase, the driver impedances are effectively in series, to match the line, and the same for the receivers.

Of course this is all moot if your signal edges are slow enough that the impedance of the line doesn't matter. If the signal rise time is several times the electrical length of the transmission line, then you can ignore proper matching with little problem.