Is there a phrase to describe two people who love each other, but are too afraid to show it?

I haven't found a general-purpose term for this. However, it is somewhat of a storytelling trope.

TV Tropes calls this trope Twice Shy, with the connotation that the two people are either too shy, too awkward, or too afraid of the circumstances if their love were to be revealed.

They also list a variation, Belligerent Sexual Tension, in which each person refuses to admit their love (sometimes not even to themselves) and takes the energy out as aggression toward the other instead.

Yes. It's called "Skinny Love":

when two people love each other but are too shy to admit it but they still show it.

[Urban Dictionary]