I don't think this is a case where you want to guess or try something and assume they'll be reasonable. Air Canada doesn't care what you put there when you sign up but when it comes to actually earning miles they're not going to award miles unless the booking name matches the account name. And when you book you'll want to have the name match your passport for fear either a border agent or TSA or someone gets picky. So you kind of have to have the Aeroplan account name match what your passport says. I suspect the answer is going to be to call Aeroplan and their agent can set one of them to be blank even if the web form doesn't allow it.