As far as I can remember, it’s the same regeneration isn’t it? I’m pretty sure it reverts back to the original Tenth Planet footage to show the first regeneration? I could be mistaken or misremembering though. I’ve only seen ‘Twice Upon a Time’ twice (ironically). Once when it was first broadcast and then again after I bought the Blu-ray. I’m probably due a rewatch.
It’s the same regeneration twice upon a time just showed that the first doctor had one last adventures after defeating the cyber men but before his regeneration. Kind of like an epilogue for the first doctor