Right idea, wrong term. Two batteries in series, the voltage adds, same current. Two batteries in parallel, same voltage, double the current. Batteries in parallel will have like terminals connected together ( positive to positive, negative to negative).
To answer the question about the alternator,
why do you need more current?
Battery size and alternator size are not really related. The battery should be large enough to run your equipement for the period of time when the engine isn't running, plus enough reserve to get you started again.
The alternator should be large enough to supply all the current you will need to power your equipment while the engine is running, plus enough to recharge your battery in a reasonable amount of time.
Let's say you need to supply 30 amps to your stereo, plus the 10 amps or so the stock engine, insturments, blower, etc. uses. The alternator would need to put out at least 40 amps just to keep up. If you sat idle for 20 minutes with the stereo on, you would have drawn 10 amp-hours out of your battery, so when you start up again you will want to replace that ASAP. If your typical ski run lasts an hour, you could get by with 10 more amps to charge the battery back up, so a 50 amp alternator would be the minimum.
Okay, now you add that second battery. This allows you to run the stereo for an hour before you are in danger of not being able to start. It also means that you are down 30 amp-hours, that if you don't replace it on your next run you will be in trouble the next time you sit for an hour.
For this sceanario, you will need at least a 70 amp-hour alternator.
Going to bigger and bigger batteries can delay the time when you are finally dead, but in the long run the alternator has to keep up. If your runs are shorter than an hour, or you spend a lot of the time idling (when alternators won't put out anywhere near their rated output) you will need a larger alternator to compensate.