I buy tomato sets at the greenhouse, they do a better job at seeding and nurturing until it is time to plant. However, there is often more than one plant per container; historically I would separate the plants and plant both but this year the roots were just too intertwined and separating would end up damaging both plants. With a pair of scissors, I snipped the smallest plant at soil level and put it in a small glass of water.
I would leave the glass of water outside during the day to make sure the cuttings had plenty of sun. Five days later:
I planted the bonus tomatoes in separate pots filled with a starter mix the next day; they will sit out this way for a week or so before planting. Should have tomatoes in 8 to 10 weeks.