It certainly doesn't look like a feud - they are posting warm messages of thanks and friendship. (However band breakups, in general, can be complex, and the full story, in many cases, only emerges well after the event)
Both are vocalists, of course, but as instrumentalists, Hannah has been best known for her mandolin work, whereas Natalie is primarily a fiddler - so there may be some change in the group's musical emphasis. (Naomi Cooke usually takes the lead singer role in RJ, and that may not change)
Natalie earlier fronted her own group - Natalie Stovall and the Drive. I follow her on Twitter, and think of her as talented, but sadly, hitherto rather under-rated. I think she will make a strong contribution to the group - and I wish all of them well. I'd be interested to see what Hannah does next. (Her message to fans says that she hopes we well continue to follow her creative endeavours - and she also refers to wanting to evolve and grow)