You can use just one coat of zinsser if you like and only 1 topcoat but for outside work and for it to look good you would probably need two
The first coat of the zinser would be the primer, the second coat would provide a decent bonding surface for the top coat to adhere , or stick to
One coat of zinsser , one coat of top coat ? ......well it MIGHT look OK but you want it to last
That's what a decent tradesman would suggest and if I wasn't replacing the tiles and wanted a decent finish and not cut corners or try to do a cheap job that's the proper process