I think if they were to try such a split, the best bet would be to do it along setting lines. Dungeons and Dragons: Eberron* Adventures (just as an example), for example, could be its own (self contained) rule book running a streamlined version of the rules focused on the core principles of that setting while being compatible with the main line. Note that I am NOT saying they would or should, just that if they did settings might be a way to avoid the market confusion aspect.
In the end, i think the folks that suggest even 5E isn't big enough to support 2 true game lines are probably right. There just probably isn't a benefit to doing so and a number of downsides. I am curious, however, if we will see any actual; new RPGs during the WotC era (a new Gamma World or whatever). I believe there was some rumor that they were working on a tabletop version of whatever sci-fi video game they are developing, but that could have been a fever dream.