I received a twinrix hepa+hepb vaccination in march 2016. is it too late to get the follow-on boosters, now 19 months later? thanks.

Probably: Twinrix vaccine is designed to be administered in 3 doses: the initial dose, followed by another at 1 month and another at 6 month. I presume your first one was March, 2015 (not '16). Some doctors might check hepatitis A and B antibodies to see if the one vaccine mounted an immune response and consider giving you just 2 more, but most would simply start the series over again, I believe. Stay well!