It looks like the SDKs are conflicting when showing the views. As I don't know much about how RevMob shows their view, I suggest simply not showing both at the same time. Instead, do the following:
When the user loads the menu screen, call the Chartboost interstitial code:
[[Chartboost sharedChartboost] showInterstitial:CBLocationMainMenu];
Then, call the RevMob code when the Chartboost interstitial is closed by implementing the didCloseInterstitial delegate method:
- (void)didCloseInterstitial:(CBLocation)location {
if (location == CBLocationMainMenu) {
[[RevMobAds session] showBanner];
}
}