Most of mine I did one at a time. However, there are some that are related that made sense to me to take on the same day. For example, I took Biology and Natural Science on the same day. I also took Intro and Educational Psychology together.
You have to decide which one works best. For me, one of the main advantages to testing and independent study was being able to concentrate on one subject at a time.

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Chartered Life Underwriter-American College 1985
Chartered Financial Consultant-American College 1986
BS Charter Oak State College 2005
BS Psychology-Summa Cum Laude-Excelsior College 2006
Chartered Advisor for Senior Living-American College 2006