Welcome, Chris.
Do you have the luggage rack already? The Triumph case will,
I think, just fit 2 helmets, and connects directly to the luggage rack. (Long since discontinued, but they do turn up on eBay.)
If you're thinking about a luggage system at some point, rather than just a top box, save yourself some aggravation and just go with the Givi stuff. E46 or E52 (@46 and 52 liter capacity, respectively) are the 2-helmet choices.
Keep in mind that a top box will hold helmets or jackets or 1 of each, but there isn't one made that will handle 2 people's gear and helmets, and if there was it would probably snap the rack right off. (I'm exaggerating, but there's a reason they limit the load to 10 pounds or so.)
I use saddlebags myself, for double the storage in (IMHO) a more aesthetically pleasing package. I also have a top box for the long trips. (Givi E41 saddlebags and E46 top case, keyed alike, with SW-Motech mounts.)
For more specific info, you might want to search and/or ask about this over in
the Sprint forum.
Cheers, HTH,
-Kit