The CSD actually gives three definitions for the word 'brank'. The 2nd definition is "bear oneself proudly, prance, strut" and the 3rd one is "dress in finery". So both these definitions would fit in with Burn's usage in verse one.

However the 1st definition it gives for the word is "behave violently or without restraint" which would fit in with Hogg's (thanks Lighter for bringing the Shepherd's contribution to light) description of the battle in his verse. I think it far more likely that Hogg uses the word in the sense that Jack suggests that it just being a typo mistake. It just one of those words which has two completely different meanings. I always thought it a bit lazy of Burns to use it twice to mean completely different things - but if the second instance was added later by Hogg then that explains it. Hogg was a wee bit lazy or unimaginative.