"Duke'' is definitely a Male Metriaclima Lombardoi(Kenyi).
His ''girlfriend'' is a pretty nice looking Labidochromis Caeruleus( Yellow Lab)
Both are ''mbuna'' which is the word used to describe rock dwelling fish in Lake Malawi,the lake both of the pictured fish originate.
They can breed together. In fact if the Yellow Lab is for sure a female which I would question anyways, they will most certainly breed. The problem is if the fish were to breed, the offspring would be hybrids,and would not be desirable,but instead would be unpredictable in both appearance and behaviors. Also the Kenyi have strong reputation for killing it's tankmates,and the Yellow lab just happens to be one of it's more popular victims.Mostly due to the similar yellow coloration.
The Kenyi will in most cases eventually either view the Yellow Lab as a competing male,or He will in the case that the Yellow Lab is female harass her to breed until she either spawns with him,or dies in refusal.
In my own experience keeping Kenyi. They need at the very least a 75 gallon tank,but preferably something with a 72''x18'' footprint or larger. Even with the largest of tanks,I would keep at least 7 females for a Lone male.Which is about double what would be the norm for a more peaceful mbuna species like the Yellow Lab.
I don't mean to spell all doom and gloom here,but I do know mbuna very well,and thats just about what most mbuna keepers would say anyways. There is always the possibility that it could work too,because the mbuna,and really cichlids in general have such individual personalities.
The scenario where it could work might be that they are both males,and are housed in an All Male tank where breeding isn't encouraged.
The Kenyi may very well become more docile in the absence of any female,and not even bother the Lab.
The other scenario would probably be that the Kenyi actually has enough females in the tank to spread his aggression,and the Lab if female would simply be one of the females in his large harem. Again if the Lab is male though,my bet is that he would become a target of the Kenyi aggression.
Nonetheless, the cookie cutter stocking lists of any mbuna tank would not list these 2 species together in the same tank,so be aware.
On a more upbeat,and less informative note. Both of the fish look very nice. So there's that.