I do agree with @the_galaxy_fox that they are protective of each other, but
Taurtis is the more sensitive one, so I think he would
-trade places with Grian if someone were try to walk off Grian
-if it's physical take as many hits possible
-get in between the person and Grian
-Knock a person out
-Or try and get Grian to laugh if it's verbal
-if he's upset comfort him
-if he's hurt tend to him
-if someone has Grian pinned, pull them off and divert their attention
And Grian is the more feral, so I think he would
-if Taurtis is upset or hurt take care of him then find the person and beat them
-If it's physical attack the person
-if it's verbal try and tell them.off that don't work fight them
-if someone were to take off with Taurtis hunt them down and get him back even if it meant blood shed.
-is some has Taurtis pinned, he would tear them.off and get down and dirty
Grian is also the one who would give people dirty looks if they were to talk to Taurtis
Also if Grian's uncomfortable Taurtis, would tell the person off then grab him and walk away
Or if Taurtis is uncomfortable Grian would give the person a glare and tear into them
So yea maybe Grian is a pinch more protective.
Do remember Taurtis is the softer and sensitive of the two, while Grian the more feral and paternal of the two