Other than being uncomfortable from having both a full stomach and a full colon, doing an enema after eating shouldn't be an issue. Digestion takes a while, so whatever you ingest, even if inedible, won't all show up in the colon (large intestine) for a day to a day and a half.
A cleansing series of two to three enemas in a row (soap suds then baking soda solution, aloe vera juice solution) will cleanse the colon effectively.
For all enemas, a quarter cup of baking soda in two quarts of water: A half tablespoon of Dr. Bronners the first one, then a cup of Aloe Vera juice for the second and just the baking soda solution for third. Water temp between 102F and 105F.