It is more important to cleanse your bowels before you take your coffee enema that after.
Personally, sometimes I do take a plain water enema after a coffee retention enema. However I always take at least one cleansing enema before the coffee enema.
The coffee acidity will only affect you if you are sensitive to drinking coffee. Normally a 2 cup coffee retention enema does not go high into the large intestines and remains in the rectum and sigmoid section. Taking a larger coffee retention enema, say of one quart and not lying on your left side, could go into the transversal colon and the ascending colon and that could cause a reaction.
An enema after the coffee enema is unnecessary. It is mainly for people who have a bad reaction to coffee.