Tape first. Use a modified thinset. It would behoove you to read the instructions with this or any product before starting your project.
What do you mean by doubling up?

I actually thought by doubling up the OP was planning on using two different waterproofing methods in the same installation.

You could be correct on that point too. This poster might be the same one that asked about poly and RedGard on another forum.

So only one. Poly or RedGard. But not both.

You were right again! lol

Thank you for the responses. Can I ask why I cannot have the Membrane for the shower pan and the RedGuard? I already have the Membrane in the pan, I was hoping for the extra protection of RedGuard so the water never gets to there (The membrane didn't go down as nicely as I would have liked).

This post was edited by netrate on Sat, Oct 26, 13 at 10:38

In general, you don't want two barriers in a structure because theoretically moisture could get trapped between the two.

If you already have your second layer of mud in place and packed in place around the drain, it's too late to use RedGard on the sloped mud. The RedGard has to go down to the drain's flange, it's referred to as "The Divot Method".

Here's a photo of the divot carved out of a mud base with the drain's clamping ring and weep holes exposed:

You can see a cross-section of what is needed in the RedGard installation instructions.

And sometimes moving pictures can be worth a thousand still pictures.

I have a problem... I told the tile contractors I wanted the hardibacker mudded and taped, then redguarded. They JUST redguarded... Can we mud and tape it after the redguard has been applied and dried, or are we out of luck and our tiles will simply now end up cracking?

As it sits now you are relying on the Reguard to prevent any movement in the CBU sheets. You also might have gaps too large for the RedGuard to cover. The Hardibacker should have been taped with thin-set and mesh first.

You can see that that I band the seams and flat trowel the entire surface. Liquid waterproofing bites or sticks better to thin-set that it does cement backer board. HardiBoard is very dry surface and dusty. If the crew did not clean it well it might not be affect so good. I have seen entire sections peel away and pre-pare my walls now first with the thin-set pass to prevent this.

Mesh Banding about 5" wide in this picture. I just eyeball the cuts.

In this tub shower the builder's crew framed the walls and cut and installed the Wonderboard for me. I will be using Ardex 8+9 on the walls and a little Laticrete HydroBan Sheet Membrane Banding to tie the tub's tile flange into the Wonderboard. But before any of that starts I mesh the joints. A 2" mesh tape is the very least you can use. I prefer 4"-6" strips which I cut from a 36" wide roll. More photo examples shown here: http://no-curb.blogspot.ca/2015/04/preparing-backerboard-walls-for-waterproofing-tools-steps-needed.html

Now when I do waterproof the seams will only get stronger and my waterproofing is not the only thing bracing these joints.

Bumping this old thread in case anyone is still around and can share how they solved for this! We have a HardieBacker install that should have been mudded and taped and then RedGarded, but instead was just taped and RedGarded, with the mesh tape embedded in the RedGard (but no thinset in the seams). Concerns are 1) preventing cracks, and 2) ensuring it is waterproof.

I need to give the contractor a clear ask for a fix here before tiling starts--what is the best solution? Slice the seams, fill with mortar, and then repeat the RedGarding once cured? Pull the tape off entirely and start the whole thing again? Mortar over the RedGarded mesh tape? If elfgirl or others are still around, would love to know what you ended up doing. Thanks!

In this EXACT same situation... except... waterproofing has been applied everywhere. So, seams were taped then Hydroban was applied right over - no mortar. What did anyone end up doing? elfgirl? artemis78? hydroban is $99/gal. SO, it would cost me $600 in materials alone to redo... no telling what labor would cost me.

I sent this in response to your PM too but for others who may find this thread--in the end we left ours as is after a long back-and-forth between our city inspector and our contractor. Ask me in 5-10 years whether that was wise--but basically at the end of it all, the inspector passed it and our contractor refused to redo it, so our only other option was to part ways with the contractor and find someone new to rip it out and start over, and also take on finishing the rest of project. That would have cost tens of thousands in our area, plus added a lengthy delay, so we decided to move forward and hope for the best. No cracks so far but of course it hasn't even been a year. The rest of the project was finished without issue and turned out well, so even if we do end up having to redo the shower at some point, we're at peace with it. (We also determined, in the course of this, that it was not possible to mortar over the tape, to slice the seams, or to pull the tape off--the RedGard was rock solid. The only option would have been to take everything off and start over.)