Remember the battery doesn't really power anything when the engine's running- the alternator will keep it charged as long as the output current is high enough. If not, you'll drain the battery over time no matter how many batteries you have.
I think trying to wire something like a set of headlights by piggybacking off the existing wiring is asking for trouble though. The 'proper' way to do it is to run a new set of wiring directly from the battery to a new fusebox. Switch the new lights from a relay triggered by the high beam wire. This makes the setup road legal as when you switch to dipped beam, all the extra lights switch off automatically. You can put an extra switch in series with the trigger wire if you want the option to turn the extra lights off entirely.