I found the solution myself. Here's my corrections:
Each term should have their own definitions of the functors (ie. the f-functor in the second term should not just link to the first f-functor in the first term, but should have its own) and pointers to the terms (a1 and a2) should point to the outermost term functor.
This means that a1 = 6 and a2 = 12 in the following layout
f(X, g(X, a)):
0 STR 1
1 a/0
2 STR 3
3 g/2
4 REF 4
5 STR 1
6 STR 7
7 f/2
8 REF 4
9 STR 3
f(b, Y):
10 STR 11
11 b/0
12 STR 13
13 f/2
14 REF 11
15 REF 15