When mining with 2 computers, do I need to use 2 separate wallets addresses?
Regarding pools: when working for a pool, the computer who solves the block doesn't credit its owner directly - it rather credits the pool's bitcoin address. The pool manager then tallies how much work each of the pooled miners delivered for the block, and proceeds to pay them proportionally to it - minus the fee the pool takes, of course.

The only exception is a P2Pool, which work with no centralized pool manager. In these type of pools, each of the peers in the pool ensure they're appropriately credited for their work when a candidate block solution arises.

As far as I know, you can configure your 2 solo-mining computers to either use each their own address or one for both. And no: even if both were using their own addresses and working independently, the processing power of the slower one is not completely wasted. Solving a block is highly random, there's not much chance both would be trying the exact same nonces in sequence. Rather, they'd each try a different set of nonces to solve the block. Obviously, in the long run the faster one will find answers more frequently than the slower one.