Might be kind of hard to stack them at all. I'm pretty sure what you need is a leader or bridal loop comming out of the top of the kite exactly opposite from where the mainlines would enter the kite if it were not for the bridal. I know it is possible to stack foils with bridals since in the late 90's one of the flexifoil team riders was using a double stack of Blades for kitesurfing.
The bridle lines/ kites might need to be reinforced to do this but since you built the kites yourself you should not have a problem once you figure out what you need to do.
If you stack the kites correctly, I don't think there will be too much of a problem with turning. Some people are stacking very large LEI kitesurfing kites of different sizes and they work well enough to be usable, though perhaps not the ultimate in high performance. They always put the smaller kite on the bottom.
Since what you are asking is a little bit unusual you should try to find sombody with real experience to ask for advice. Maybe try: