I'm not aware of any Mega-specific commercial shields, other than for larger LCDs. You could make your own starting from a blank proto shield.
The simplest way is to bend out, or clip, the A4,A5 pins on the RTC shield and run a jumper pair to 20,21. It is anybody's guess as to what the RTC shield is, but it may already have provision to re-route the connection.
Pic shows DS3231 RTC on small board which also has Bluetooth on Serial1.