Yep, I know, one of them used to draw the top half of the fish and then another would finish it off with the bottom part.
But what is the message behind the usage of the fish symbol itself...
Go and fish for people, i.e. win people for Christ; spread the Gospel; baptise the people, heal the sick, etc.
The acronym or so-called abbreviation was only thought out much later by some fertile mind. The church in the middle ages always tried to add some mysticism to Cristianity. This was the power base of the power-hungry priests.