mixing cement with plaster?
It works well. Dry mix the cement and plaster first - before adding water. Success really depends on what you’re doing with the mix. For instance, are you casting? troweling? With just cement and plaster you have no aggregate, like sand or gravel or binders, fibers, etc... so your structural performance will depend on several other factors.

If you want to strengthen plaster you should first determine the type of strength you require: surface hardness, durability-resistance to abrasion, improved compressive strength or impact resistance. 

Fibers of polyester, glass, nylon, metals are all added to enhance performance of concrete and all of these work well in plaster. There are plasticizers which improve flow through molds. Not unlike adding manure, straw, horse hair or grain chaff to clay to stucco, adobe and daub.

One of the easiest strengthening additives is plain white glue. Test a small batch.