It’s illegal if they do. But I’m sad to say, it happens.
A lot of people get Lasik in one eye in late December and in the next eye in early January so they can use a full year’s amount for each one.
Besides, going out for a New Year’s bender with two eyes that see very differently is going to make for much more interesting bobbing, weaving, lurching, and hurling.
It’s a win-win situation if ever there was one. 
I think they invented them so employers could profit from your healthcare savings.
Because clearly adding another level of profiteers to the mix is the fix America’s disastrous healthcare system needs!