It's a Monday so I'll really step in it. People like to defend their choices here.
From a safety standpoint, I'll take a two battery/one alt system over a two alt/one battery system any day. This assumes of course that both are designed and maintained appropriately. Have fun with is one.
Yup, it's Monday so I'll do the same and jump in. I think just the opposite (Unless of course you have full EI, in which case you pretty much have to have two batteries.). I'll take a backup alternator any day. In over 2000 hours I've already had three alternator related failures (never the B&C alternator itself), and no battery failures. It would be an interesting poll to see how many folks have had battery failures v.s. alternator failures....
Apart from master solenoid failures, which generally occur on energizing (Contact corrosion, wear), not random drop off during flight, loss of a battery seems far, far less likely than alternator failures. Alternators have many ways to fail - to start with, they could be a Plane Power (joke!), brush wear, broken field wire from engine vibration, broken belt, diode failure to mention a few. By comparison, batteries are far more robust wrt sudden failures. So with one alternator and two batteries, with an (more likely) alternator failure you can just fly a bit longer due to the additional battery charge, and if its a backup battery that may not even add up to much. With a battery failure, ok, you could just keep going, possibly having to switch things around accordingly.
With two alternators (one the backup), with the far likelier alternator failure than battery failure, you can keep going with the backup. Done that, all the way from Teterboro to Chicago. And with a (highly unlikely) battery loss, apart from the issue of not knowing and dropping the alternator due to a sudden substantial load change, you also keep going.
That brings us to the issue of detecting a battery dropping out and in most cases going undetected. I dealt with that with a sensitive battery ground strap current direction detector. The battery ground side cable should always have at least a small amount of current flowing down toward ground. Flowing upward means the battery is not charging or discharging in excess of alternator output plus loads, and no current means the battery is failed/disconnected. The current sensor looks for that positive current to frame ground, and if it is negative signals a charging/alternator failure and if it is zero, a battery failure. And in the worst case fly home with the battery backed up Garmin G5 and handheld!