Frequency selective power limiter
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:
input --> circulator <--> LPF --> low freq output
\---------> HPF --> high freq output
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.