Well, you're starting to get a consensus on the tubing size, so that's good.
I'm not sure why Stenner is shying away from using a Y, unless they're thinking that you're thinking of using a barbed connector, then I would agree. Don't use one of those. You
are increasing the number of connections, so that technically increases the risk of leaks/failures, but a typical acid injector system has several such connections. The mini-check-valves I suggested add four more connections. So there's that.
And while the plumbing world is moving more and more to shark-bite type push/quick connectors, I don't trust them myself because there is an o-ring involved, and I've had them pop off on me. I would use compression fittings throughout, because that's what the IntellipH system uses (and Stenner, too, I believe) and if tightened correctly they won't leak or pop off. The Pentair mini-check I suggested appears to have compression connectors as well.
Something like the following, although I would want to see some spec's on whatever fittings I use that speak to their resistance to whatever fluid you're pumping. Pentair uses the term "Corrosion Resistant." The Pentair mini-check I suggested is "Corrosion Resistant" but that part might be intended for chlorine, so I can't speak to how it will do with acid.
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Respectfully, I don't agree with Dave's other comment either: "What you're wanting to do amounts to a suction side manifold." It's not that. A suction-side manifold uses one pump to draw liquid from multiple pipes. What you want to build is two pumps drawing from a single pipe, and that adds some complexity, namely the two pumps drawing from both the source and each other. Which is why I suggested the check valves in front of each pump. If the Stenner pumps you want to use are peristaltic, then I suppose they might supply the anti-backflow capability that the mini-check valves will. I'm not sure of that. I'd probably use those mini-checks either way, and as close to the tee as possible, especially if the distance between the tee and the pumps is more than a few inches.
Again, just spit-balling with you, as I don't think anyone here has actually tried this before. We're still waiting to hear what you're trying to do with this rig. If you're pumping into a pool, I'd be tempted to try this out with water first (NOT pool water), before connecting the injectors to a pool plumbing pipe and before adding the acid (or whatever fluid you're pumping), just to see how it will all work, specifically if the injectors' output is satisfactory for your purpose. That would also be a good way to measure and determine the flow rate: how much each injector outputs per minute of pump runtime: some numbers that might come in handy later on when computing your dosing.
