FOL substitution - is it possible to substitute two variables with each other? e.g. $\theta=\{x/y,y/x\}$?

Question

Let $C = m(P,X,Y) \leftarrow m(Q,X,Z), m(R,Z,Y)$.

Is it possible to do the following substitution?

$D = C\theta$ where $\theta = \{Q/R,R/Q\}$ s.t. $D = m(P,X,Y) \leftarrow m(R,X,Z),m(Q,Z,Y)$