DDH, you read my mind!

Anyone using dual batteries (such as two 600 cca batteries instead of one 880 cca)? It is probably a cheaper way to get more juice ($29-$39 each for the cheap ones, $59-$69+ for one that has a lower cca than the two combined.
Any idea how much the alternator is capable of (relative to battery rating or number of batteries)?

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1985 300CD 279k
2000 F250 powerstroke