Thanks for the feedback, Robie.
I've since read, as you suggest, that blending yeasts in a single fermenter is not a good idea for the reason you state. In the future I'll be sure to separate if I'm using two yeasts.
I don't recall where I got the idea to use two yeasts. It might be in my notes at home. But when I read it I misinterpreted what was meant by blending yeasts. I also read that some wineries do actually do single-fermenter yeast blends, but they surely know a whole lot more about what they're doing than I do.
According to the Lallemand website, D254 has a "neutral" competitive factor, whereas Syrah has an "active" competitive factor. I'm thinking that if they're both neutral, you could blend them. I think I figured since neither were "sensitive," they might both be able to get the job done. In any event, I had a rapid start to quick, healthy fermentation. I don't think there was much lag time where the yeasts fought it out before getting to work. I did pitch them into different areas of the fermenter and didn't stir them in until the first punch down 12 hours later, at which point a cap had formed.
I probably won't do this again any time soon, mainly because what good is experimenting if you don't have a control to compare it to? I have no idea what impact blending yeasts had, since I don't have wine made from the same grapes but only using single yeasts. Whatever the result, it's all a lot of fun. The wine is coming along well. It just finished MLF this week and I'll be racking it onto some oak spirals tonight. It still has an MLB smell & taste (kind of cheesey), but if you look past that it's jammy, fruity - blackberry and blueberry - with big, big coffee notes. I'm looking forward to the final product.
But for now, I'm also looking forward to the fall harvest... and finding a source for premium wine grapes!