Although you can use two uncoupled single ended transmission lines for differential signals, that's neither the usual nor a practical way.
Consider that the effective differential impedance is formed by the parallel circuit of differential (wire-to-wire) and single ended (wire-to-ground) components. In PCB design, space saving demands to minimize trace separation, the single ended component will be respectively higher than 50 ohms, e.g. 70 ohms. Check by comparing typical single ended and differential trace calculation results of a PCB tool like Saturn toolkit.
Apart from the discussed point, 45 + 55 ohms S.E. involves an unwanted imbalance, but is probably roughly within design margins.