Take the t-shirt you want to use for the majority of the dress. (this is the section under the bust.) Cut off the shirt sleeves and cut across the top of the shirt removing the collar. Turn the shrt inside out and stitch up the slits where the sleeves use to be
take the second shirt and cut off a section of the bottom hem that is big enough to cover your bust. Attach this piece to the bottom piece that you just cut off. The shirts should attach at the spots you cut leaving a hemline at the bottom and top of each dress.
cut a small slit in the hemline of both the top and the bottom. Measure out a piece of elastic big enough for your bust and another piece for around your legs where the bottom hem will sit. Attach a safety pin to eash elastic and feed through the hemlines. Once they are fed through stitch the top elastic together and the bottom elastic together and TA-DA You're Done.