Games : Can I use a Master Seal to promote into a previous class' promoted class?

My Robin is nearing level 20 and still a Tactician. Let's say I want to promote him into Grandmaster eventually, but I'd like to pick up Armsthrift first and take advantage of the base class' superior stat growth.

Can I Second Seal into Mercenary and then Master Seal into Grandmaster, or do I have to go back to Tactician first? If so, is it a good idea?