i can tell you, i have had a hammer for about a year. started as 6 heads. i fragged 4 off and sold them... so i was down to 2. in a years time, i now have 8 full heads (more if you count them splitting). the pic eb8919 posted is exactly what mine look like when they split.
first you will notice the flesh under the actual tentacles will split. also the mouth area becomes elongated.. kind of like a cell splitting.
in the end, you will end up with 2 heads.
unless... your frogspawn is weird like my hammer. my hammer has 2 heads now... going through a 3 way split.
to complicate things even worse, you will sometimes see small buds forming off of a head. these are babies... which will become a full head in time, and will break off (or you can break them off) and start a new colony.