Legal Question in Business Law in California

Interesting question. Probably no laws that specific. Probably some general laws governing fair business practices, ethics, competition, etc. would tend to restrict you from working for competitors at the same time. Also, if your agreement entails making a full-time effort on behalf of Company A, you cannot legally also pledge a full-time (or even part-time) marketing effort on behalf of Companies B, C, or D, etc. I'd suggest re-reading your agreements with each MLM company for specific promises you've made to each.

Only through common sense and reality check.