If in each of input-port the frequencies are different, then you cannot obtain the interference signal from the output-port. But you can get a beat signal.
If the two inputs have the same amplitude and they are phase locked(You can find some references of phase locked between two laser beams), the beat signal will be perfect.
If there is no phase locking but the frequency difference is much bigger than the linewidth of lasers, you still can get the beat signal but not so good.