Just to clarify, I'm not suggesting that you plug into two 120V circuits to charge the Tesla.
But.... a "proper 240V line" is two 120V circuits. That's how you get 240V, you use two 120V circuits that are on different phases (residential US circuits are two phases).
What that means is an electrician can take your two 120V lines (if they are on separate phases which they probably are, or swap one of them at the panel so they are on separate phases), and wire them to a single 240V 15A outlet. With a sub panel, you could also have some 120 V 15 A outlets. That is how all 240V circuits in the US are done. ...