I believe you should look for "gangable" medicine cabinets if you want multiple cabinets to work together. I'm planning to do 2 Robern PL series cabinets to make an approx 36" span of med cab above a 40"-ish long vanity in a small bathroom. This involves buying a robern 49.99 ganging kit to combine them (that's the list price, not the price actually paid on the ganging kit). That's the only gangable/modular Robern series I can fit into my budget (comes to $660 including ganging kit, not including tax).
Slightly cheaper option #2 was a Kohler Verdera 2 door, 34" one piece med cab (cost is $440).
I believe the Kohler Catalan line of med cabs are gangable, but too tall for my bathroom and would be pricey like the Roberns anyway.
My hall bathroom currently has a mirror that spans the width of the vanity but it's very impractical (for me) not to have the storage of the medicine cabinets there. I could live with just one medicine cabinet, but I prefer more mirror. But yeah, medicine cabinets can get pricey.
Q