so have decided to my buy a fighter for my little tank.

but is their any way i could put two inside ? with a jar or something or wont the jar heat up?

The water in the jar would get toxic with ammonia etc. very quickly. I had one in a large breeding trap for a while when it was getting hassled by a gourami, but it wasn't happy in there.

Maybe make some kind of a divider for the tank out of a mesh if you're determined to keep two?

A breeding net would probably be ok But remember the quality of life a fish can have stuck in a net or jar all day..

I did this for my fighters, i had plastic jars and drilled holes in them and hung them on the side of the tank. Every week i let a diferent one swim around in the main tank and they seemed to be happy.