Hi Andrew
The short answer to your question is that, except in the case of very open grained/porous woods, you should only use one coat of sanding sealer. Although this shouldn't be a problem when using a wax over the top of it, there can be a problem when overcoating using a lacquer.
(Relatively speaking, sanding sealers are softer than lacquers; if more than one coat of sealer is used they can move against each other as surface coatings and being sandwiched between the timber and a hard lacquer they will craze, like a laminated windscreen hit by a stone.
The most annoying part of this is that it can take a long time to happen, so what looks great today could look awful in (say) six months time!)
A second coat of sealer in your case won't really achieve much, and I am a little confused as you are looking for a tough finish but using a wax as a topcoat. The sealer should be tough but the wax will not be!
I would agree that some more information would be helpful!