You can use a 2mhz crystal and the gameboy will run almost half as slow.
But the problem is that your songs will sound out of tune. Since the gameboy uses the clock speed to determine the pitch.
For example. A C4 note at half speed will be a C3
The problem with a 2 MHz clock is that a C4 will be just a little lower then a C3 that way.
Which could be interesting tho
And you can fine tune instruments in lsdj, but that's a lot of workaround.
So in short. Yes a 2mhz crystal will work, but your gameboy will be out of tune.