I imagine that Percival Graves comes to know the Scamander brothers when he was still a new-in-the-rank Auror in MACUSA who was sent to UK as part of delegation. He met with the elder Scamander first then was introduced to the younger Scamander who was at that time not an auror but was still working in British MoM's Office for House-Elf Relocation (I read that in wikia).

I imagine Scamander brothers as a unit who shares everything. Theseus might be a more popular and more social wizard, viewed as the less strange one of the Scamander family while Newt viewed as the strange one because of his preoccupation on those magical beasts.

But anyway, I am okay with every imagination you can conjure up on them ;)