I have 2 x 2 cylinder diesel Yanmar engines. They are in a catamaran that has been sitting on the hard stand fro 5 years. What do I need to do to before I start them?

Replace with new any rubber pump impellers you might have in the raw water cooling system.

First thing to do is your fuel tank it may have Sam sturdy disease
It may have need of some biocide in the fuel tank or tanks if there's two tanks
Personally I'd drain the tank and put fresh fuel inland add some biocide
I'd flush the fuel lines up to the filters and replace the fuel filters
Bleed the fuel system
Drain the engine oil replace the filter
If it's not raw water cooled drain the coolant and after inspecting the water pump impeller after its drained
Also check the gearboxes or sail drive read the manual if it has sail drives
Before starting it I'd make a mess and cracked open the injector fuel lines and then wind the motor over
Doing that will give you two positives
One fresh fuel right through the system
Two would winding it over you will be getting oil pressure before it will be started

If the engine is sea water cooled I'd drain the engine block refit the drain plug and pour in some vinegar for a few days

Thanks for the advice, I am prepared for the expense, just do not want to compound it by stupidity. Good advice here. Cheers.