It's likely they are the same sex, often times males are territorial and combative especially if a female is present, but even female can be territorial and combative to other females. Either way you needed to separate them immediately when conflict was noticed, I hope you are not to late, separate them before it's last turtle standing. Below is a pick of my pair of diamondbacks, male on the right with the smaller head and shell but longer,thicker tail cloaca an inch past shell, the female is on the left, small short tail with cloaca right at shell and larger shell and head. Some other water turtles will also have differences in front claw lengths and plastron differences.



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Jeff Benfer
1.0 cinnamon pastel Python regius
1.1 pastel Python regius
1.1 mojave Python regius
0.3 normal Python regius
1.3 Terrapene carolina thriunguis
2.3 Terrapene carolina carolina
4.1 Kinosternon baurii
1.1 Malaclemys terrapin terrapin
2.2 double het albino and anerythristicThamnophis sirtalis parietalis
1.0 anerythristic Thamnophis sirtalis parietalis
2.3 Iowa snow Thamnophis radix
0.2 het Christmas albino Thamnophis radix
1.1 double het cherry erythristic, albino Thamnophis sirtalis sirtalis
1.1 melanistic Thamnophis sirtalis sirtalis
2.0 66% het snow Thamnophis sirtalis sirtalis
1.1 triple heterozygous for amelanistic,carmel, and stripe Pantherophis guttatus
0.1 anerythristic motley Pantherophis guttatus
0.1 butter p.h. stripe Pantherophis guttatus
0.1 carmel stripe p.h. amel Pantherophis guttatus
0.1 amelanistic p.h. carmel,stripe Pantherophis guttatus

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