I have a file with
:- [library(clpq)].
eq(X1, X2) :- {2 * X1 + 3 * X2 =:= 6}.
then I compile and run and I get:
?- eq(A,B).
{B=2-2 rdiv 3*A}.
It's the result you're expecting?
edit
?- eq(A,B),A=1.
A = 1,
B = 4 rdiv 3.
?- eq(A,B),B=1.
A = 3 rdiv 2,
B = 1.
Section A.8.3 of the documentation says that unification can hold 'outside' constraints specification. Then you can experiment freely with additional bounding. But if you impose that both A and B will bind, you should choose appropriate values. AFAIK the values you posted initially cannot satisfy the equation.