That’s pretty intense phosphate reduction. Why the desire to mix?
That aside, phosguard doesn’t need to tumble and Rowa does. For that reason, I wouldn’t mix. You can go for much higher flow with phosguard but you’d have to dial it down to make sure the Rowa is in a gentle tumble.
My suggestion: pick one and see how it does. I’d start with the Rowa in small amounts, you don’t want it to reduce phosphate too quickly. Rinse well. I’ve used both, phosguard is amazing but after an ICP test my aluminum was elevated and the only suspect/variable was Phosguard. They say it won’t leach over a certain PH but that wasn’t my experience.
Just my $.02!