Both players have units stacked in the same hex, but are not in close combat.
I think the rules are sloppy/unclear/inconsistent with their "close combat" terminology. I believe that the intent is that if both players have units stacked in the same hex, then by definition those units are "in close combat", regardless of whether units within that hex have attacked (via a "close combat attack") each other yet or not.
(E.g. an example in 18.3.1 Barbed Wire says "A Soviet Rifle wishes to move into the German hex in order to initiate close combat.", and the descriptions of several cards like Seek Cover talk about a unit "in CC".)
One of he players fire to them from another hex. Are all
units affected by the fire or only the enemy units?. I know the answer is yes if the are in close combat but can't find this specific case.
Firing into such a hex will surely affect all the units there, regardless of whether or not a "close combat attack" has already occurred in that hex.