Anyone know how vision insurance benefits work?

I am status indian and therefore get basic benefits, $100 of which is my coverage for glasses(not contacts). I also have employer benefits which covers me for $500 for vision, both contacts and glasses. My contacts cost $130 for 6 months so to get contacts for 1 1/2 years would cost me $390 but I still wear glasses as well. Would I be able to get coverage of $100 from my indian affairs benefits and then use the remaining $110 from my employer benefits to get a $210 pair of glasses? Can I stack both coverages or can I only use 1 at a time?