First, I work in iOS, not android, but the concept should be similar. You would need to use the delegates so that you could display the second after the first is dismissed. I didn't use revmob, so you need to see their documentation for the delegates, but if you wanted the reverse order (chartboost->revmob), one way to do it would be:
- (void)didDismissInterstitial:(NSString *)location
{
//this is fired after a chartboost interstitial is closed
//so you set the code to display the other starting from here.
}