I personally keep a single standard (non-cross) hotbar visible near my cross hotbar for skills that I have on my extended or w (double-tap) hotbars that I want to keep track of (mainly oGCD skills).

Different hotbar layouts are going to suit different people. You're going to want to flesh out your bars in a way that makes you comfortable. However, it does help to look over others' bars to get a feel for how you might want to plan yours out. To that end, here's a document containing all of my (tentative) cross-hotbars. The Red Mage hotbar is near the bottom.

For reference: "1" and "2" are hotbars 1 and 2 (I can tap RB to toggle between them), "W" is for the W-cross bar (double tap LT or RT), and "Ex" is for extended bars (RT+LT or LT+RT). Cells colored purple on hotbar 2 copy the action slotted in hotbar 1. I also use the alternate way of visualizing the bar, where the d-pad slots are always left of the center and the ABXY slots are always on the right.

As for explaining my RDM setup: I keep most all of my (useful) spells and essential oGCD skills on the primary bar. Situational and less-often-used oGCD are in the right extended bar, and the left extended bar is for "junk". Hotbar 2 is my "melee mode". When I'm ready to spend the mana gauge I press RB and gain access to my essential melee skills.