I've lurked here for awhile but I haven't seen anything quite like this, so I thought I'd give this tutorial thing a try. Many thanks to the rostitchery tutorials ([urlhttp://rowena.typepad.com/rostitchery/techniques/index.html[/url] )for inspiration
Tonight I turned these

into this nursing top:

Sorry about the flash and crummy pic. Here's a pic showing the nursing opening (you pull the top part up and bottom part down):

Here's how I did it:
I started out with the two shirts and seam ripped them up both sides all the way to the armpit.

Then I cut the flowered one across from armpit to armpit to make a rectangle. In the pic, you'll see I actually made a little divot in the middle of the top so it wouldn't show under the deep v of the top part.

Then I gathered the entire top of the of long sleeve tee in the middle and used one of the straps from the tank to secure it:

Note: this made for a lot of fabric over the chest. Next time, I think I will cut about 1/3 of the fabric off the bottom of the orange tee, rehem the bottom, and then do the gathering.
Next I gathered the edges of the top tee so they reached about halfway down the back and basted them to the back of the long sleeve tee. Here's a pic of my pinning for basting

and one of me trying it on over a different tank to see how it fit (this would be where I should have noticed that even my giant nursing boobs are no where near a size 44EEE so I probably should have used less fabric -LOL)

Going back to the flowered tank rectangle, I attached a piece of 1/4" elastic across the top, stretching slightly. Ignore the horrendous sewing job

Finally, I sewed the flowered rectangle onto my orange tee (orange tee was turned inside out):

Done! I'm not sure that I like the two different fabrics...I think I might like it better with two matched shirts, but this is what I had lying around and I thought it would make it easier to explain the process.
Note: if you don't like the gathered top, you could also just cut off about the bottom third of the top shirt, resew that hem, and then sew the underlayer on as above. It would make a Boob-style top like the one shown here:
http://www.duematernity.com/bovrinutop.html.