Maddy, I have not tried this but having grown lots of tomatoes and lots of lettuce, I don't see why it would not work out as long as the lettuce matures and is ready to eat before the tomatoes get about 2-3 feet high and start to obscure the light to the lettuce. You may want to leave a little bit of extra space between your rows of tomatoes to make it easier to pick the lettuce. You could even include a few basil plants in the lettuce row as others suggested. Lettuce can be planted a bit earlier than tomatoes because it is more cold tolerant, so check the "days to harvest" for your lettuce varieties and plant them in the tomato bed early enough for them to mature before the tomatoes get huge. I am not sure where you are located, but in my USDA zone (6) I would plant the lettuce in early May and put the tomatoes in in late May, then harvest the lettuce in June before it gets very hot. Lettuce will not grow well in your garden in July/August (usually it bolts to flowering right away), although some varieties may be resistant to bolting (check your seed descriptions).