in regards to having the glass parallel or slightly angled - my mate recently approached a glass manufacturer about this when building his studio.
if you're using proper sound proofing glass, you don't actually need to worry about doubling up. proper sound proofing glass is in fact created by vacuum sealing 2 sheets together. The 2 panes don't actually touch, and there's no air between them (vacuum), so it's silent (sound cannot travel without a medium).