OK thanks for your help!
Unfortunately x and y don't reach a/b at the same time, as can be seen in this new figure with a=0.05, b=0.2 (so a/b=0.25).
So with this solution, I have the equilibria (x,y,u,v)=(1,0,0,0), (x,y,u,v)=(0,1,0,0), but not (a/b, a/b, 0,0)...
What would be great would be...