I'll take a stab at this, with no practical experience with this product. Their website states that it is best to apply it directly to squeaky clean paint (it has no cleaners). They state it can be applied over wax, but as the inferior wax degrades, the Rejex will disappear with it.
Liquid Glass is an old school polymer. Without knowing its properties, you may or may not have the same results as applying it over the wax mentioned above.
I see no advantage or logic to applying it over LG. What's your reasoning on this?