@Thibaut, for your reference, here is a Playground that animates the top and bottom halves of a mesh separately: https://playground.babylonjs.com/#S0VVNV#53
Please click inside the Playground scene to activate pointer lock.
Pressing the following keys will start the animations for the bottom half:
W: Run Forward
A: Run Left
S: Crouch Backward
D: Run Right
SPACE: Jump
The top half automatically cycles between Rifle Aim and Punch.
Please check out the Question thread where this was discussed: Animation/Blend Trees? (Part 2)
This thread mentions how the mesh was split into two (top and bottom halves), and @MackeyK24 showed his awesome progress on bringing Unity animation blend trees to Babylon!