Suppose that $\theta = (\theta_1,\theta_2)$$\theta = (\theta_1,\theta_2) \in{\mathbb R}^2$ are the parameters of a model, and that I can obtain a MCMC sample from the posterior distribution of $\theta \mid {\bf x}$.
Using the MCMC sample, how can I obtain the conditional posterior distribution
$$\theta_2 \mid \theta_1, {\bf x}\,?$$