It isn't possible for two signals that have different
frequencies to have the same phase. They might be in phase for
an instant, but because of the different frequencies, they
cannot stay in phase. However, you might divide them or mix them
to a common frequency to produce square waves that are in phase,
out of phase by a fixed amount, or at least near the same
frequency. These can be compared to produce an error signal
which can then be used to pull one of the frequencies relative
to the other so that the phase relationship is maintained. This
is called a phase locked loop. This is commonly used to produce
a range of stable frequencies from one stable, but expensive,
reference frequency like a crystal oscillator. Not sure how you
would do this with lasers, but I guess the principle is the
same.