A co-owner can be sponsored for an H1-B. I have seen it done. But you have all the normal H1-B issues e.g., prevailing wage. Also the word from immigration lawyers is to expect it to be much harder for small startups to get H1-Bs for any of their employees this year ... very bad news.

The O-1 class of visa is also an option, no prevailing wage requirement. So that is more attractive for a startup employee/founder. But you have to meet the "outstanding" requirement. H1B and O1 apps cost the company applying approx $5000.