Sure, it's possible. You will need to know how to merge portrait files and edit the cultures files though.
Check which cultures use the same GFX in both and for those take the faces from Tubbs and the clothes from BLG. For where the cultures have a different GFX, create new ones and manually merge faces + clothes.
'A Game of Thrones' is a user modification for Crusader Kings II not affiliated in anyway with Paradox Interactive or the rights holders to 'Game of Thrones' and 'A Song of Ice and Fire'.
This modification and it's team takes no income or profit, whether it be through donations, advertisements or any other form of revenue.