Thank you for the responses everyone! Luna does trot easily in hand. I've had her pretty much since birth and have been ponying her from Stella since she was 6 months old. She lunges, backs, flexes and many other groundwork exercises. When ponying her in the past I've just always only walked her around our property over obstacles and short walks down the road. Only in the last couple of months I've been trying to get her trot along with us.
Stella responds well to my leg and will slow her gait/stop when I shift my weight back without any pressure on the reins. So when I get Stella moving at a trot and Luna refuses, she pulls me back in the saddle causing Stella to slow and then stop.
But my biggest problem is the steering and you all are so right. I need to get Stella neck reining before adding Luna again. I guess I knew that all along and was in denial. I was looking for a shortcut and I know better. Thanks again!
Stella - sweet, timid, elegant, lovely, lively, amazing
Luna - large, unattractive, naughty, adored