Good Question;
I've done some biax with 600g plus matt backing and I've found it difficult to work sometimes,so now I'm looking at two layers of 400g biax no matt.
On the other hand we bought a different biax cloth 600g no matt that was very soft,it went on easy but was a little too soft to handle and would be better with a layer of matt.....so there is the conundrum...
It depends on the manufacturer of the glass,check it out and ask ,Noahs in Toronto has nice firm glass,no matt so I'd go two times 400g(nm) in that case as recommended by Rick at Noahs....
Is one method stronger?
No, the one layer of 800g would maybe use less epoxy.....by a bit ....but they both are equal strength .
It's important to get the second layer on soon after the first (still green) with the 400g before you need to sand .
Overlap the ends of cloth adds strength to the bond rather than just putting the second layer right over the first....if it is possible to overlap that is.
So I lean towards the two layers of 400g,both sides thats how I'm going on our Eclipse build (3/4in C.C. foam/glass/epoxy) ,the Bridgedeck roof will get done that way later on.
Either way depends on you ,your helpers ,size of piece ,temperature ,and what you will be most comfortable with as well as make of glass....
Hope I havn't talked too much...;0)