Agave and Shrubs

Well, you wouldn’t plan a plant combination in one planter where some plants required dry soils and dry crowns while their neighbors required moist soil.. so there is no horticultural reason to consider two mulches (aside from the design aspects). Shrubs do not require organic mulch as such. Depending on the shrubs, they may require a more organic soil than Agave… but again, that makes them bad companions for agave.
Match your plants to the conditions and the mulch to the plant needs and aesthetic.

The mulch (any mulch) serves to retain water, keep down weeds, and visually tie the planting together. With some plants, an organic mulch can hasten stem or crown rots… so ill advised for agaves, etc.
Use the stone/marble/recylced glass/glass eyeball/ball bearing/whatever inorganic mulch of your choice on agave and shrubs alike. Keep in mind that inorganic mulches may get hot in hot sun, so your shrubs must be OK with that, like the agave is. Make sure the planter drains well and the soil mix does too. As long as your “drought tolerant shrubs” (that can mean many things) have the same requirements for soil type, soil temperature, and moisture as the agave.
Good luck Adam!

Agave, like most desert plants, are indigenous to soils that have very low levels of organic matter. They tend to prefer it that way in cultivation. In my experience, agaves in platers can be affected by heating of the soil, particularly if it has a southern exposure. Of course it get much hotter in Las Vegas, where I am, than Thousand Oaks. Here we typically use rock mulches as organic mulches get blown away and disintegrate quickly in the dry climate. As has been noted, reflected light from rock can burn the underside of leaves on some shrubs.
Using two mulches would be a waste and not likely to be kept that way over time. That is, when it is time to add mulch, one type will be used.