Double-Gasket Exhaust Manifold?
 

Installed test pipes ~2 weeks ago, and used Z1's multi-layer metal gaskets. Frustratingly, the seal of this product was extremely lousy and there was a visible gap between the gasket and flange (neither flange is warped).

I ordered a set of Fast Intentions gaskets to remedy the issue, but while the design looks great I'm still somewhat worried they are too thin and I should order a second set to double-gasket the flanges. Has anyone done this before?

Generally on the flat exhaust flanges you want the thinnest exhaust gasket you can get (assuming your flanges are clean and flat). Adding thickness is band-air for another problem, or likely to be a failure point in my opinion. If you have a gap, something isn't flat, or something is misaligned. You may need to loosen the rear sections before tighten the front.

I'm running the gaskets from the FI, no leaks here. I personally don't see a reason to add a second gasket.

Too thick of gaskets can be the cause of a leak, especially on a 2-bolt flange. As you tighten the bolts it can crush the gasket right at the bolts and the flanges can bow and not apply the same pressure across the whole flange. If the flanges are perfectly flat, have a clean surface texture, and have ample surface area contact... sometimes I dont even use a gasket, I just apply a thin coating of copper sealant. This can keep the contact pressure even and prevent the flanges from warping when torqued.