I've heard of people using the stock under-floor mechanical clutch pivot and hanging a dual-master brake swingset on the firewall.
As long as you plan ahead so the pedals are spaced comfortably and don't need to pass through each other to function (duh...) it won't matter that one goes through the floor and one goes up to the firewall -- that is unless it matters to your sense of aesthetics.
I'd think the biggest worry would be having a big wide brake pedal that binds on the clutch pedal/arm.
