Jonothon,
Terminology between here and the US can vary, so can you define what is meant by "safety glass". Tempered glass is heat treated in a way that causes the glass to break into small squarish pieces, ( imagine car windows). This is frequently referred to as "safety glass". over here in Aust.
Laminated glass is is actually two sheets joined by a thin, flexible membrane, (this is done by the manufacturers) and is preferable for studio windows as the membrane helps reduce resonance in the glass sheet. Obviously the thicker the better. There is available, a laminate glass with a slightly thicker membrane than is standard which was apparently designed for acoustic use.
I would suggest using 10mm (3/8") laminate on the tracking room side and 6mm (1/4") laminate on the control room side. Mount the glass in the frames using neoprene strips to isolate/ insulate the glass from the frames as much as possible AND make sure they are totally air tight.
It is also wise to put a quantity of silica gel crystals in the void between the windows to absorb any moisture in the air.
According to local glazing companies, tests have shown that using two differing thickness sheets is more efficient at reducing sound transmission than angling the windows, although I personally wouldn't dismiss the idea............cover your arse and use both methods
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