If your PIX is not "heavily loaded" then it's OK to use the same interface for both of these. The LAN interface is used for config replications and regular health checks. The stateful link will be much busier usually, but what you don't want to end up with is that the link is so saturated that the health checks can't get through and the units start failing over cause they think things are bad. The issue is exactly what is "heavily loaded", for which I really can't tell you an exact answer.
