Actually I think those sound rather perfect as a sibset - they both have the same no-nonsense image. They go together much better than, say, Tiffany and Martha, or Margaret and Crystal.And Maggie/Meg/Margie goes well with Martha too!
They're similar, but I wouldn't say they're so similar that it would be tacky to use them together. Especially if at least one of them has a nickname that sounds more different.