I earn both from BTCGuild. Does this take away from my bitcoin mining speed? I hope it doesn't.
No, it won't affect your speed. You hash the block headers in exactly the same way as for normal bitcoin mining. The merging is done by the pool. Simply said, the pool constructs a block that will provide proof of work for bitcoin, but the same block also proves namecoin work. This is possible because namecoin is designed to piggy-back onto a bitcoin block (there is a very small overhead to the block size, but the merging benefits both currencies). Its explained in more detail here http://bitcoin.stackexchange.com/questions/273/how-does-merged-mining-work