That connection between them is the Storymind itself. It’s the part that can do things for a while (climb over mtns) and then consider another way (walk around the mtns). That’s why the stories don’t have to be connected through storytelling. Their spatial and temporal arrangement within the same story mind are enough to connect them.
There is something else to consider in this sort of experiment, though. If your storytelling between throughlines leaves them obviously separated, you are counting on the Storymind and the audience to connect them. Some of the audience probably won’t, but some of them probably will. The Storymind will keep up with it just fine as long as the structure is solid.
BUT if your storytelling makes the stories look like they’re supposed to be connected through storytelling and yet you don’t show your MC in some way relating with your IC, you’re probably going to have some storytelling issues that get in the way of your message. If your story is about Frank the policeman chasing Jim the bad guy, and the storyform would have Frank influencing Jim to turn himself in, and yet they never meet and Jim has no knowledge of Frank or his actions and yet Franks steadfastness pushes Jim to change, you have a problem. Not a structural problem, necessarily, but at least a storytelling one. “Why would Jim turn himself in if he never even knew that Frank was willing to sacrifice something important for the greater good?” the audience might say. Even if your Storymind gets the message…even if the audience gets the message…you have broken the storytelling by not giving Jim the info he needs in order to change. You’ve broken the internal logic of the story world.
So just because you CAN keep throughlines separate and still pass on the message doesn’t mean your story won’t appear to be in some way broken in the realm of storytelling.