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 :eek:
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.