Someone asked this question elsewhere and I'm curious to know what this is.

It's about a boy with poor health who lives with his grandfather, and when he goes out to pick flowers for his grandfather he ends up crossing a barrier (into kinda like another dimension/world) and finds this man laying in the field. and they have these two mirrors that let them see each other no matter where they are.

Tsuki ni Ookami maybe

Chapter 2 and 3 of Koi wa Maiorita

Thanks, cl123.