Besides the obvious (brake pads, different tires and tube etc.) what if any are the issues with running a clincher Zipp 404 on the front and a tubular Zipp Disc on the rear?
You can use normal brake pads with the Zipp disk so that is not an issue - and other than the question of why mix 'em - why not?
Why not? The only drawback might be if you plan on bringing spares, you just have to bring a tubular and clincher tube too, otherwise, not really an issue.
I would second what JM said… a spare tire, (or two) and tubes is a lot of crap tp haul around.
Other than that What advantage is there to runnign wuch a set up? I really can only think about negatives
Other than the fact that it’s a royal pain in the ass? No reason. There certainly aren’t any safety concerns (that I’m aware of) such as you’d get when mixing radial and bias-ply tyres on a car… But as other posters have mentioned, you have to carry 2x the backup materials to get you home if you flat out. If you’re racing a flat IM course and those are the best wheels available to you, it might be worth it. But you’ll have to decide on the tradeoff between running fast wheels, and carrying a lot of junk with you.