I could take all the lessons and practice my whole life and never get a sound like they have.
IMO, you don't really want their sound, if, what you're looking for is to tap your talent/skill- develop your own sound using their music as a guide for what you want to accomplish- a style- all your own-
Willie is an anomaly- I read, years ago, somebody in his family (not sure who, or if it's even true) told him he was a pretty good guitar player, but a lousy singer- years later, a who's who in recording wanted to record with him and many still do- want to. So much for others opinions. Right?
His style (and Waylons, for that matter- among others who preferred doing things their way created a whole new genre in Country music- Outlaw) - they didn't like the empty suit approach to homogenizing their music- voila! Outlaw- Kris Kristofferson is another great example of style (and writing talent) whose voice leaves a lot to be desired, but, he had style-
BTW, I also read that Willie lamented one time, it took him 20 years to become an over night sensation- Blue Eyes Crying In The Rain did that- then he could seemingly do no wrong, musically- and Bobbie was beside and behind him all the way - she's a great pianist- watch that video and watch, as well as listen- they both really get into what they're doing-