start riding your bike with one hand, and holding the second bike to the side of you at arm's length.
KEEP YOUR WEIGHT OFF THE SECOND BIKE. try riding forward slowly.
if you put any weight on the second bike it will start leaning over and steering against you.
pretend you are riding one handed like you practiced, and holding a heavy bag off to the side with your other hand. lift up on that bag, don't push down.
the second bike will be perfectly upright if you are doing this well. if it leans to one side or the other or feels like it is steering against you: briefly lift the front wheel of the second bike off the ground - this will get it back on track and keep you honest about having your weight only on the first bike.