Why is it suggested to apply two coats of sealer?

Applying two coats of any sealer will dry and create a better finish much better than one thick coat.

These sealers are water based, and they cure and form a film primarily during the evaporation of water from the coating. A thin coat will evaporate much faster and create a single uniform layer. A second thin coat will fill in areas not filled by the first coat and create a deeper, richer, more uniform coat. Single thick coats cure less evenly, have a tendency to puddle, and are prone to tracking.