Consider the TRS R consisting of the rewrite rules

1: f(f(x)) -> g(f(x))
2: g(g(x)) -> f(x)

There are 2 dependency pairs:

3: F(f(x)) -> G(f(x))
4: G(g(x)) -> F(x)

Consider the SCC {3,4}.
By taking the polynomial interpretation
[f](x) = [F](x) = [g](x) = [G](x) = x + 1,
the rules in {1,3}
are weakly decreasing and
the rules in {2,4}
are strictly decreasing.
Hence the TRS is terminating.