I will certainly start to move toward you method. I've also read comments by professional builders to always have 2 kits going on simultaneously. The plan to finish off some of my shelf queens this coming year. The stash is getting ridiculous.
The few professionals I've known usually have more then 2 builds going, as they're on a time schedule per contract/agreement. Most modelers I know for years on end have a few builds going at one time, but are at different stages, so that if you're lets say priming one aircraft, afterwards you work on another build during your work session.
I've just never been able to do that and keep focused on a build through completion. The few times I did try that, I found myself reaching a difficult area of the build, and just stopped working on it. The reality is that's when I need to really focus and work my way through it.
For me, my method works. For you, find what's your comfort zone and stay within it.
As for stashes, my max is 12 kits. Never more. I never worry about a kit going out of production, and not being able to find it later. With For Sale sections on virtually every modeling site, and ebay, you can find anything you really want. Besides, with the constant increase in kit accuracy and greater detail, a new release of the same subject happens more often then not. Same for AM parts.
I know several guys who literally bought 2 and 3 of every release in the 70s and 80s figuring that what they didn't build or want, they could sell for a profit once the kits went out of production. As it turns out they spend all their time at shows and contests taking a table or two, and trying to sell off what they can at a loss from the literally thousands of kits they still have.