It might work,how big is the CK? I have had afers tear smaller fish up at night but with bigger fish they seem to get along and ignore them.I have had afers together in a tank with no problems between them,in fact they would hide together until the lights went out so the two may or may not get along,not sure what to make of the bichir.
Best to get the Ck bigger than the afer as if the same size the afrer will harrass the ck. Especially u already got the afer. Afer will usually go after newly introduce knifefish unless it's 2-3x the size.