If one failure would be reduced braking and the other no reversers for example, you couldn't even be sure adding the reductions to each other is enough.
The percentage reduction in for example braking deceleration might even multiply with each other resulting in a reduction greater than the sum.
You would really have to talk about the specific MEL item to even make an educated guess how the RTOW reductions interact with each other.
Edit: Bad example, reversers aren't used for RTOW calculations (i think) but i hope it's clear what i mean.