This an interesting one. The question was for "guaranteed" differences between Cichlasoma bimaculatum
and Cichlasoma dimerus
. Anal spine number is a good one; being a member of a clade is less easy to see.
But I wonder why the choice is/was between these two species. After all you bought them as Cryptoheros myrnae
. In the genus Cichlasoma
there are another ten candidates for your fish. And maybe even a few in Aequidens
too. Any pictures?