You are sort-of answering your own question. Yes, we can make better things, and that is normally what we try to do. If no-one is building a supercar that is powered by two large V-8 engines, it is quite probably because, for one reason or another, it is not better. Technically, it is certainly possible, and has been done in the past:
[ame="http://en.wikipedia.org/wiki/Railton_Special"]http://en.wikipedia.org/wiki/Railton_Special[/ame]
Or why not use three engines?
[ame="http://en.wikipedia.org/wiki/White_Triplex"]http://en.wikipedia.org/wiki/White_Triplex[/ame]
but again, if it is an old idea, and no-one does it now, there is probably a reason. A simpler, and arguably more charming way to achieve that kind of power would be with a single aero or boat engine, like this:
http://www.telegraph.co.uk/motoring/...ley-video.html
and as you can see, it is hardly a supercar, in its characteristics.
The Cizeta-Moroder had a 6L transverse V-16, with central power take-off. It was about as narrow as a V-16 could reasonably be, and yet it still resulted in one of the widest cars ever built. Regardless of the raw dimensions of an 8L big block engine, it would be impossible to fit two of them transversely, end to end, in a road legal car. Fitting them longitudinally, side by side, would be problematic, with regard to finding space for exhaust manifolds, and arranging the power delivery from two separate shafts. Mounting them in tandem longitudinally would be possible, but would result in something with the length, and handling characteristics of a yacht, not a supercar. The advantages of a single larger engine include the lower polar moments of inertia, as the weight will be more concentrated.
In addition to the size and weight of the engines, there are also structural considerations to think about. The transmission would have to comprise either two gearboxes, or one very heavily reinforced gearbox, driving through a very heavily reinforced drive system. The chassis would have to be very stiff and strong to resist the torsional and flexural stresses of the power and the weight. Finally, what type of tyres and brakes would be suitable for such a heavy and powerful vehicle? As always, weight is the enemy, and ultimately, the excess weight would give unfavourable handling characteristics, and blunt the performance. It would be fast, but probably only as fast as a smaller car with a single big-block engine.
I am also rather more attached to small, light, high-revving engines. Something like a pair of Yamaha R6 engines would give a 1.2L V-8 with about 180kW, and a red line of about 16,500rpm. It might not go so spectacularly fast, but imagine the noise.