Stearic acid is a thickener. It will make your lotion thicker and your soap harder but it will not keep your lotion from separationg.
If you are looking for an emulsifier you need emulsifying wax or beeswax.
I tried a few lotion batches with stearic acid and cetyl alcohol ... It doesn't feel that great, imo.. if you have it on hand and are just practising then go ahead, but I'd call it inferior compared to others.