Not bad. Not ideal. As you suspect, the bottom card and the PSU will be fighting for the same air FLOW. There's plenty of air for both, but the direction of air flow may be towards the more powerful fan. However, the bottom card is usually idling and doesn't get warm until heavy gaming in SLI aware games. If you are able to monitor your card's temp during those demanding times, you can tell if the situation is critical or not. Monitoring the PSU temp is a bit more difficult.