You need some frequency-selective component to be able to discriminate between the 2 signals at different frequencies.

LPF filter is the most general solution (works over the widest frequency range).

If the frequencies are known in advance, a notch filter or harmonic filter might work better.

The quarter-wave stub idea is a type of notch filter to create destructive interference at the frequency you want to reject. You could use a tuneable quarter-wave stub if you need the filter to be tuneable, though it'll only notch out one frequency at a time.

A diplexer is a 3-port filter, with a common input and 2 outputs: LPF + HPF, typically with less insertion loss than splitting the signal and filtering each signal individually.

You can make a poor-man's diplexer:
Though you're relying on the LPF to reflect the out-of-band signals at the input port, which isn't always the case for LPF filters.