just putting them next to each other and tieing them together doesn't graft them together. all you have there is 2 plants tied together, still 2 plants not 1. Even if you were to really graft them, they would still count as 2 plants, not just one plant.
If you really want to graft them however, go to your local nursery and ask them for some grafting powder. After you get the powder you have to cut both plants down the stem to reveal the soft insides. rub the grafting powder (of liquid if that's all they had) and press the two together and wrap them up so they stay together. It will probably take a few weeks for the plants to securely attach them selfs to each other. There is more to it, but the people at the nursery should be able to explain it to you.
Like I said, this doesn't make one plant. It gives you two plants that are essentially glued together.