I made these grazing boxes (or grazing frames as ive been calling them) for my birds and they love them. Ive have always just planted grass in them, but im probably going to plant both grass and herbs this spring. If you want to plant both grass and herbs, I think that you may have to make two separate frames. The frame with the grass will need very small hole wire. This gives just enough space for the grass to poke through without the chickens sticking their heads through the wire. If you want to plant herbs (or any other leafy green like lettuce, kale, or spinach) you would have to use wire with larger holes. The larger leaves need a bigger hole to be able to poke through the wire. The reason I say plant them in separate frames is that if you mix the seeds together and use large hole wire to accommodate the larger leafy greens, the chickens will be able to poke through the holes and eat the grass without allowing it to grow first. But if you plant both and use small hole wire, the leafy greens wont be able to make it through the wire and cant be eaten.
Wire like this for grass:
Wire like this for herbs:
But this is just my opinion on this. Im sure mixing the seeds will work, but just not as well as keeping them separated in different frames.