More than two outboards for sailing catamaran?

Apologies for any confusion Catsketcher. My point of interchangeability is to have a full complement of engines at all times, properly equipped for their intended use. Hypothetically, if the mothership had 3 Honda bf15s, and the tender had 1 such, in the event of an engine incapacity on one it would be a relatively simple thing to remove an engine from whichever boat had less need, and transfer it (with the new, appropriate prop) to whichever had greater need until repairs were effected. Limited spare parts required, simple redundancy.

 

Okay - still unless the boat is special it would be hard to get an outboard that could do both jobs well. I have seen an Oram 44 with twin 20 Tohatsus on the cat and one of the dinghy but usually you will have a hi thrust on the cat and something fast and lightweight on the dinghy. It really is not that important a facet - in 10 years my Yammie stopped once - because of dirty fuel. There are more important design criteria that should top this.

 

In practice it is not possible to choose theoretically optimum propeller and gearing. If it would, then the optimum is both large diameter and large pitch with suitable gear ratio allowing the engine to achieve recommended rpm range. Yes, Pitch/diameter for the optimum case really is more than 1.5 for all propulsion cases, not just for fast boats, but even for tug boats when moving (but not for static thrust case).

In practice you should begin by researching the market for what is available, including all available propellers for each outboard you might consider. Just because some dealer or manufacturer calls an outboard high thrust does not really mean anything at all, a big mistake assuming so.
For example Yamaha F70 regular version has gear ratio of 28:12=2.33 exact same as FT50 or FT60 high thrust models have. If the same propeller selection fits them all ( I do not claim that being the case, please check it out for yourself!), then F70 regular provides the most torque at the propshaft, allowing most thrust as well.
Yet F70AETX is the lightest of those 3 outboards at 121 kg, while FT60GETX weights 127 kg.
A single F70 has far less propulsive disc area than 4 FT9.9 combined, resulting far more slip and lower efficiency with more fuel consumption even at the same speed. It is however capable of even more speed with even greater fuel consumption as it has far more power than 4 times 9.9.
The prop selection for FT9.9 is excellent including, but not limited to 11 3/4 inch diameter, with pitches: 5 3/4 , 7 , 9 1/4. All being dual thrust aluminium props.
It is pretty much guaranteed one of all the available props will fit 2...4 FT9.9 outboards if used on your boat. The same can not be said for all other outboards, possible not even those marketed as high thrust models.

Yamaha FT25 is a four stroke engine with 2 cylinders, bore 65mm stroke 75 mm, 498 cc.
FT25 2017 Features & techspecs - Outboard Engines - Yamaha Motor UK https://www.yamaha-motor.eu/uk/products/marine-engines/portables/ft25.aspx?view=featurestechspecs
What is the available propeller selection for Yamaha FT25?
A quick search only find one size at 12 1/4 x 9. This pitch might not be the correct for your case.
What is the available prop pitch & diameter range for Honda BF15 or BF50? And gear ratio of those?
Any outboard of any brand, for which a suitable prop is not available will not be the correct choice for your boat! Check available props before purchase!!!
And as last point, you should estimate intended travel distance under power for life time usage, to determine if any saved fuel justifies any additional purchase cost or not. Or if any extra weight of heavier outboards will be fully or partially counteracted by less fuel weight carried along due to more efficiency (mostly propulsive due to more combined disc area, not blade area or differences in ICE efficiency as those are far less).

 

Concerning the price for the Yanmar outboard diesel, this web page estimat

Concerning the price for the Yanmar outboard diesel, this web page estimates it will be in the $25,000 range.

Yanmar To Sell 50hp Diesel Outboards (Video) https://www.passagemaker.com/technical/yanmar-diesel-outboard

I think this price would put it out of consideration for a lot of people.