I do. I actually mix a bunch of them: henna, indigo, shikakai, amla, and sometimes aritha for extra cleansing. (I'm waiting to throw cassia, brahmi, and bhringraj powder in the mix when I get them...
) Each powder has its own benefit, so instead of doing separate treatments, I just mix them. I figure I've had good results using them alone...they're all powders....why not just mix 'em?
Although some of these powders can be drying, I only add them to hair saturated in oil, and the mixture is mixed with 40 parts water and 60 parts oil (I make enough for about 6-8 washes) to off-set the dryness.
It works great, I highly recommend trying it.