Mixing the two assures good conditions for the microbes and fungi that actually decompose the plant material. The smaller the plant materials are, the faster they will decompose. Shred, chop, or mow before putting them in the bin. Moisture and air are required for the composting process, but too much or too little of either one can cause problems. Compost materials should be about the wetness of a wrung-out sponge.
Anaerobic bacteria dominate wet compost piles and create bad odors.
The solution is to add air by turning or mixing the pile. If too dry, the materials will decay extremely slowly. Keeping the pile covered is one way to control moisture levels. Turning every week or two mixes the materials, speeds decomposition and is a good way to monitor progress. Finished compost is well worth the small amount of effort.