YES

1 decompositions
 #0 -----------
   1: f(a(),x) -> f(a(),g(x))
   2: a() -> b()
   3: g(x) -> x

@Simultaneous Critical Pair
--- R
   1: f(a(),x) -> f(a(),g(x))
   2: a() -> b()
   3: g(x) -> x

--- S
   1: f(a(),x) -> f(a(),g(x))
   2: a() -> b()
   3: g(x) -> x