It's not a myth - there's much truth to it. I personally have caused a precipitation whiteout in my tank way back when I was a newbie and didn't know about (or possibly ignored!) all the oft-repeated, drilled-into-our-heads admonitions against dosing alk and Ca at the same time. But, the way I see it, that goes back to a time when dosing pumps were expensive and not readily available; many dosed 2-part in daily, weekly, or even less frequent boluses... people were strongly advised to add one 2-part supplement, wait a while, then add the other with good reason. The outcome from dumping in a good quantity of alk and calcium at the same time is not pretty
But when using dosing pumps (for me, it was old Alitea peristaltic pumps originally, and later, the DŌS when it came out), there's a key difference from doing single larger manual doses - typically one is dosing very small amounts at a time, and those tiny doses get diluted almost instantly. For example, my current tank gets 33mL/day of alk supplement; that's dosed in minuscule 0.3mL doses every 13 minutes. I've yet to see any indications of precipitation happening when using my DŌS, even during those rare periods when I was dosing equal amounts of alk and Ca, and therefore the dosing of each was at the same times. As an additional measure, I do as you do - position the outputs of the pumps into an area of relatively high flow; it's a simple, common-sense thing to do. IME, there's no need to go to great lengths (as some insist on doing) to have complicated programming or create umpteen separate intervals in a DŌS schedule.