carry (something) onto (something)
To bring something (often luggage) with one onto a vehicle. Because it's only a weekend trip, I'm just carrying one bag onto the plane. I wouldn't carry that much stuff onto the bus during rush hour—you'll be lucky to find enough room just to stand!
carry something onto something
and carry something onto take something onto a vehicle. Do you plan to carry this bag onto the plane? I'd like to carry on two bags. Can I carry them both on?