Theoretically, yes, it will increase your power, but it will be difficult to use both motors at the same time. One way that I could envision would be to hook up a chain drive with one motor at each end and a sprocket that would power your drive shaft in between. The problem is that, practically speaking, this is a very inefficient system. The motors lose power by driving the chain, the drive shaft will need to be supported by extra bearings so that the middle sprocket doesn't float (extra friction), and the rotary shafts on the motors will be pulled to the side by the chain, etc. While you will probably get more power than either motor could output on their own, you'll end up losing a lot of power from each of the motors due to inefficiency. You'll probably end up with only 1.5x the power, not 2x.
If your problem is price, think about how much more work it will be to rig both motors to pull together, the price of the extra parts, and all the extra room it will take on your robot. I think getting a more powerful motor would be the way to go.