Two main gun weaker than one .
And the real object 490 was different animal.
There were
several objects 490. None of them were built anyway(including 490-I you're probably talking about; there were 3 others, apart from 2 I've already mentioned). All of them reached more or less the same standard, hence a detailed mockup (on photo).
Basic concerns(salvo fire/rapid-fire, retaining full combat capability after a hit on the gun or even ammo stowage) aside,
placing two guns on the flanks w/o intrusion into turret volume simplifies internal layout significantly when your whole crew sits in the turret: you don't have to deal with the need to independently rotate crew stations, they simply sit in non-rotating central ”drum“ within the turret. Placement of all optics thus becomes as simple as it may possibly be, due to the very same reason.
As a result, you gain the lowest possible armoured crewed vehicle with the least technical complications, where the central part of the turret(the likeliest place to be hit) is actually the strongest: no gunport.
As a useful bonus, guns themselves work as side protection of sorts for the fighting compartment.
This layout (among others) was contemplated by people with a lot of experience behind their belts: there were no obvious show stoppers.
Too radical to be build? Perhaps, but not stupid.