To attain their goals of balance, handling, packaging, etc it HAD to be a boxer style engine...I think I remember seeing one of them mentioning a Porsche and Toyota collaboration.
Say 300hp was the spec on the gen 2 86, that involves a massive amount of changes that the OEM has to deal with, and that means R&D to make sure the product is safe, functional, and reliable...exponentially more involved than what an aftermarket manufacturer would ever do.
More Power means the clutch has to support that power, which means stronger transmission internals means physically larger or more expensive material, then larger axles, larger bearings, bigger ring & pinion, stronger driveshaft, heavier components mean beefier suspension components, which means bigger bushings, which means bigger mounts, which means more structural reinforcement at those mounting points, more stress at those points means more chassis modifications, etc, etc.
Which makes everything heavier, and more expensive.
To add in a second engine, means compromises to please both power plants. Compromises are expensive, and less effective. I wish American companies did this single engine option more, invest in a really good power plant and perfect it...not change it every year to appease brochure browsers.
There was a lot of work from Toybaru to create this platform and keep it economical...adding motor options does not fit their goals....basically the answer to them was Miata.
@
Goingnowherefast none of those motors are candidates although the K series and Skyactives are great motors!...they either physically don't fit in their platform for an OEM, and/or won't pass current emissions....and S2000 engines are not bulletproof to 700hp, not even close...1JZE/2JZE maybe yes. And the Renesis is pure garbage...that is a blehhhhhhh motor in a great chassis.