I managed to resist by reminding myself that I have tons of products already, most of them are liquids and creams (which mix easily), and I used to play Makeup Mixologist on the back of my hand all the time. So let's just go back to my roots and save $27!
Daylight on left, flash on right.
Here are two of the combinations I really fell in love with. The first is equal parts MAC Strobe Cream and Becca Beach Tint in Dragonfruit; it creates a glowing coral shade that looks--surprise surprise--a lot like the swatches I've seen of Becca's Lychee/Opal mix, just less golden. If you wanted to create more of a soft pink highlighter, you could easily add a far smaller amount of the Beach Tint to your white highlighter for that effect.
I also wanted to see if I could add a smaller amount of highlighter to a cream blush to make it softer and more dewy. The BBIA Downy Cheek blushes are already quite creamy, but if you want a more pastel shade with even more glow, you can add a dab of MAC Strobe Cream, blend the two, and get the effect.
If you really want to add a powder to a highlighter and see the results, you can, but one of the products should still be a cream or liquid, and loose powders are obviously easier to use. Also, you don't have to use flat white highlighters; other shades can work.
Here's an example using Kismet Cosmetics Toasty highlighter (top left) mixed with MAC Jardin Aires pigment (top right--squint to see it). Mixing the two makes the highlighter more golden and reflective. The bottom left shows the mixture swatched heavily, which I think would make a gorgeous eye color, and the bottom right shows the mixture sheered out, which is a slightly lighter, more glowing, and more golden cheek or brow highlight than Toasty is on its own.
Obviously, you can still apply your blush, then top it with a bit of highlighter. That's a stand-by routine that I'll probably use for most of my makeup wearing days. But mixing the two products together, than applying, really can give you a different effect and provide you with a wider range of colors to choose from. As an added bonus, if you really enjoy a mixture you make, you can pre-mix it and save it in an airtight container from a store like TKBTrading.com.