MTurk: can 2 people from the same household join the site from the same computer

I do not think so. Now if it is the same MTurk that uses Amazon, you might have joined as Mr. and Mrs. ---- and that would mean your earnings are joint. But usually you and her have to have different ip addresses and if you want to make twice as much money, that means another computer. Of course, she could act on your behalf by using the computer while you are at work, but I have no idea if that is legal. It might be if you were disabled and you could not use your hands, then she would be using the computer on your behalf. The best thing is to write them and ask them.

You can only have only one account per household as per the rules and regulations of the site. If you do not want to get banned I suggest using only one account. I hope I did provide you with a solution.