Although I haven't built a Porsche motor yet (and will be asking lots of questions from you guys when I go back up), I would think stacking gaskets would not be a good thing.
I would think the movement between the two while you torque would cause uneven distribution of pressure. I know in the jet engine world, we were never allowed to stack shims or gaskets of any kind.
But, I'm not an Engineer and have never torqued a 911 cylinder down, so what do I know? 
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Cory - turbo'd '87 C3.2 Guards/Blk, 3.4, 7.5:1 CR, 993SS cams, Borg-Warner S366 turbo @ 1.2-1.5 bar, Treadstone full bay IC, 70mm TB, TiAL F46 WG, HKS 1 1/2" BOV, twin 044 pumps, MicroSquirt AMP'd w/GM smart coilpack, Bilstein coilovers, Tramont replica Speedlines (285's rr, 225's frt), Big Reds frt, 993 rr., tower brace, MOMO wheel
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