Yes, 2 8 ohm cabs equals 4 ohms, and that's true either way you connect them- both from the head, or daisy chained- same thing, parallel. And yes, you add the wattage of both cabs together. But don't worry about overpowering, because the wattage ratings on the cabs mean very little. Yes, you can blow those spkrs, but that's true of any cab. Just watch your low eq and volume, and listen for spkrs "farting out". You should be fine.