I'd keep the 21, and sell the 18... and cough up a couple more bucks to get another 8 ohm 21sw152. Wire both cabinets in parallel and run them on one amp.
I’d still use one amp per woofer. Amp is rated [email protected], [email protected]
So if you run one per amp, each driver gets 1050w
If you run two per amp, each driver gets 775w.