There are three rules that come into play here. One has to do with the maximum load you can put through a conductor. Another has to do with the selection of a breaker, based on the load. The third has to do with the connection of more than one conductor to a breaker. As long as the total load through any one of the 500 MCM conductors does not exceed 380, and the total calculated load on the breaker (100% of the non-continuous plus 125% of the continuous) does not exceed 400 amps, you will pass the first two tests. But I can't imagine a breaker that has double lugs or triple lugs for 500 MCM conductors. How do you plan to make the connections?