We tested this in closed beta, and actually the second console gave a cumulative bonus. In other words it was :
base + (base * bonus1) + ((base+bonus1) * bonus2)
no diminishing returns.
Hence, we got some huge bonuses from stacking two big damage bonuses of the same type in CB.
We were getting absurd plasma cannon damage. Since you can have plasma torps and cannons, you can use +plasma to boost them all...
I know from testing it in OB that it no longer gave strictly cumulative bonuses. Someone mentioned that they are now based on the CO formula for stacking bonuses, though I can't remember the math. (should be able to search the archives for stacking. It was about a week into OB where the thread came up)
Also, they split projectile bonuses and energy bonuses, so +plasma energy weapon console no longer has an effect on plasma torps or mines, but there is another console for the plasma projectiles (again, tested in OB)