I mix all the time, never had any problems with it.
- Karoo rear, MT21 front
- CO2 rear, 6Days front
- Scorpion trail rear, Trailmax front

In the case of the first 2 it was a conscious mix, because I like how they work together.
For the third it was due to my Scorpion Trail front getting slick ambitions. The Trailmax was the only decent tyre I could get on short notice in Slovenia. I figured it would be better to mix than to ride into the coming thunderstorms with a bald front. The rear was just new from before I left on holiday and I did not count on the front wearing so fast. It hasn't been a problem though, they work well together, even though they are very different tyres.