Upon doing more research (I'm still new), it seems like what I'm imagining is called ground layering. If I were to ground layer a juniper tree from a low-hanging branch, could I keep the new root system/plant attached to the first tree?

You COULD, but are you trying to grow a multiple planting? For one thing, all bonsai design, whatever the tree(s), rocks, anything, is always recommended to include odd numbers, as the results are usually more interesting than e.g. a double or quad planting. Just curious why you'd want to have the new tree rooted in two places... what's the objective?

You can do exactly what you were asking about. Many raft plantings are established by ground-layering, and I use it regularly as a propagation tool in the raised beds I have devoted to bringing along bonsai material.

When we establish bonsai plantings, odd numbers are usually considered more eye appealing, with the exception of twin trunks; so it's suggested that plantings with 1, 2, 3, 5, 7, and 9 trunks are preferred. Beyond 9 trunks, your eye is probably unable to discern between odd/even, so any number of trunks greater than 9 would be fine.

When you layer your juniper, get everything situated, then strip some bark off the bottom of the deepest part of the layer. Allow at least a year for rooting, longer if the plant isn't in good health. If you want to use a rooting aid, use only IBA - other compounds can hinder rooting.

Al