Slitherine

Hey Y'all, So I have a technical question that is time dependent. My army is trying to break another army of mine that is under siege. The enemy army I am facing is big and I can only guarantee success if the besieged army helps me by joining me in the battle.
1. Can this even happen where both my armies join up THEN attack the Other Army?
2. If so, how do I make this happen?

Thanks in advance everyone, this is a nailbiter of a turn so I would love someone to respond

V/R
George

Click on the garrison unit. There should have an option to exit on the taskbar that appears. Click on it and the garrison will exit and join the battle. However, I don't remember if your two armies fight simultaneous or sequentially.
I hope that this helps.