Not many marinas have two 30 amp plugs.  That's when this Y adapter comes in handy. It mates perfectly with the EEL cables we have on board for shore power: http://amzn.to/2rW51kv

A few specs:

• 50 Amp 125/250V Locking Dock Side/Male Plug, Two – 30 Amp 125V Locking with Sealing Collar System Boat Side/Female Connectors
• EEL (Easily Engaged Lock) is a patented, industry first, jaw-clamp design providing one-handed operation and a secure waterproof seal which completely eliminates the need for a locking ring
• The EEL connection is built for the harshest environments and is UL approved
• Allows you to split one dock side receptacle between two boat inlets of similar or dissimilar power configurations
• UL approved