You can physically merge the two collection lists using javascript, however it’s a little difficult to imagine what end result you’re wanting in this. You may need to sort the elements in some way as well to blend them together the way you’re imagining.
That depends on what you need it to accomplish, but we’re talking about straightforward DOM manipulations. Element moving, sorting, etc. If you need a dev to help build it you can direct message me and I can send you my rates.