It is not necessary to use only Seal-Lac prior to Royal-Lac application.
Seal-Lac is a shellac based sealer and pore filler. It is always a good idea to use a sealer before application of any kind of finish. A good sealer acts as a interface between the wood and the final finish. It not only gives a smooth flat foundation for the finish but also helps in adhesion. It is highly recommended that Seal-lac be used. However just plain shellac can also do the job. The only difference being that it will not act as a fast pore filler like Seal-Lac.