What is the best method to attach 1/2" backerboard to 1/2" drywall for shower tile application? I know this is not recommended but dimensions have left me no choice.

Man, you're full of ill conceived plans tonight :)

Tile requires an incredibly stable and solid platform. More so for a bathroom application, where the moisture will exascerbate any shortcomings in your job.

If you're going to do it, glue & screw. A thinset or mastic between the drywall and backerboard, with screws every 8" on every piece of wood you can find behind it. Tape the seams and thinset.

Then hope for the best.

Thanks,
Json

Yep, "Tape the seams and thinset" and use a nylon tape on the seam. I use acrylic modified thinset for bath tile on backer. It is very important that you get the right type of screw and hit the stds every time. James Hardie Company makrs a nice backerboard and they sell a screw called a Backer-On. It taps the backer well and as the head sets it makes its own countersink.