Say one Chaos war band wants to run a training simulation against another war band to build on tactics and unit cohesion. Would there be an area where they could fight like its the real thing without consequence?

no.

I think that a training ground should be in the game for each faction.
After all the SM for example have training facility wherre they can train and get used to each other. Sometimes those training ground are also adaptable with weather changes and sometimes captives xenos.
Maybe could it be somewhere a battlefield out of the campaign map for "free" fights.

we will have some safe place where maibe we can test loadout and class but not a combat simulator against member of our faction

"Oh look, Friendly fire is on."

"Hey what are you guys doing?"
"Testing things."
"The battle is over there, any you want to kill farm echother?"
"Yea so we know what is the best equipment sets we can use."
"That's fine an all but you do know that every enemy as different weaknesses, that Plasmagun will do great against slow heavy infantry but be nearly worthless at CQC against fast enemy right?"
"I guess we should just use what is best for the situation, not try to min max."
"Good on you *clears throat* FOR THE EMPEROR CHARRRGE!!!"

Something akin to a firing range/training room onboard guild strike cruisers would suffice though.

I like the idea of duels between same factions. Wolf vs Angel

I really liked Planetside 2's Virtual Reality Training Zone. It had NPCs that you could test weapons on and it was a large enough zone that you could easily practice with an outfit should you need to. (There wasn't really any need to, but it was nice to have that option)

The best testing zone is in a real fight everywhere else is boring

How about they actually go and fight the enemy? It's not like you lose anything except time if you die.