I am 100% with your wife when she states that she WANTS a twin motor.
I sold a lot of ribs with twin engines, and I was probably one of the 1st in France to do that, ie persuade the customer that 2 is safe, 1 is not !
I sold italian ribs, but most were south-african, a country where nobody takes the risk to go with a single engine. There a white sharks everywhere, so what could you do if your prop starts to slip or breaks ...?
I would strongly suggest a pair of 90-2 stroke like Yamaha - extremely reliable, I had a pair on my own 632 Tomcat - and I fitted more than 15 SA cats with 90 and 50 either.
Yam 90-2stroke not being counter-rotating, you will have to ask - possibly insist - them to be installed with a slight 3° vertical angle - ie not vertical - that is with the point of the V upward.
When this is done, the skegs act like foils.
This is important to get both motors within the same RPM, and avoid the 2xprop torque to push the port pontoon down.
This angle is simply made by drilling the top holes straight in line, like if you wanted the motors vertical, then screwing the motors with 1 hole difference between left and right screw.
On the cats, this angle was 7° (2 holes) - otherwise 6000rpm on right engine, and 5000 on left !
If you choosed counter-rotation, vertical.
Don't be afraid by the statement "with 2 motors, 20% of the power is lost".
This becomes true when you install the max. power allowed by the hull, because the weight of the single becomes much lower than twin.
With 2x90 Yam, your 6.5 Ballistic should reach 40 knots, probably more.
And with those light motors, the dynamic behaviour of the boat will be much better than with 100kg more behind the transom
Should you have other questions, don't hesitate to contact me directly