Translation:They split up yet they loved each other.
I think the French sentence is not synonymous with your They split up although they loved each other, but rather with Although they split up, they loved each other. In addition, grammatically, the conjunjunctive adverb (and) yet in Duo's translation, which often behaves like a coordinate conjunction, seems to me to be closer to the the French (et) pourtant.
This is more a question of English grammar rather than French. ''Elles s'aimaient'' can mean either ''they loved each other'' or ''they used to love each other''. With the context of the French sentence there is a clear implication, mostly due to the word ''pourtant'', that they split up at a fixed point in time, in spite of the fact that they still loved each other at that fixed time.
The English sentence ''They separated, yet they used to love each other'' implies that they separated at a fixed point in time, in spite of the fact that they loved each other for a period of time BEFORE that fixed point in time, however no longer loved each other at the time of the break up. So it is an unnatural sentence; the translation given makes a lot more sense.
"Separer" to separate thing a from thing b...conjugated with avoir. similar to 'aimer'. "Se separer" to separate from each other..conjugate with etre similar to s'aimer(se aimer) to love each other. Les médecins ont séparé les jumeaux...The doctors separated the twins../avoir/ no reflexive pronoun// .. Les jumeaux ont été séparés par les médecins. The twins were separated (got separated) par les médecins. passive voice with ETRE but AGAIN again no reflexive pronoun se...//Les jumeaux se sont séparés après l'université. The twins separated after college...reflexive pronoun SE. **Note that as one poster coyly noted here,Duo is using the feminine plural here to refer to TWO females.
Absolutely makes more sense to say "They got separated/they split up... EVEN THOUGH they loved each other".
If "yet" must be used, then it very easily makes more sense when the sentence structure is "They loved each other, YET, they split up."
Unless you're trying to say that "Even though they split up, they (continued to) love each other".
I guess it just hurts when Duolingo 'separates' you from being marked correctly, even though you're clearly still in love with being marked correctly. # deep sigh
One problem is that the two French past tenses, the imparfait and the passé composé don't map directly to corresponding English past tenses, so it can be tricky to translate.
The imparfait is often combined with the passé composé to show relative times or to set the scene and then describe an action in that timeframe.
In this case "They split up yet they loved each other." (at the time they split up) would be quite different from "They split up yet they used to love each other." (but no longer at the time of the split), and I think the French as given may be needed to achieve the former meaning. It would be interesting to get input from an expert.
... and "They are separating (from one another / they seperate (from one another))" = "Elles se séparent"
nbbarthy - please note that "Elles sont séparées" means they are physically seperated from one another by someone/something. See dbguy49's earlier comment for clarification.