May I ask what kind of friendship/relationship do they have? Thanks in advance!
They don't have any overt direct relationship, so it's more like trait connection.
Both Teio and Venus Park/Treve are heralded as the geniuses of their generations (both umas and IRL horses). They also closely follow the footsteps of their "paternal" umas/horses; Papa Rudolf for Teio and Grandpa Montjeu for Venus Park.
Both characters also have similar color schemes and somewhat similar personalities.
All the above has led to the fanbase joking about Venus Park being "French Teio".