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The external rusting is considered normal. Here's a comment from Subaru Canada:

"On newer vehicles, some customers may complain about a premature rust appearance on their exhaust system, mainly on the piping.
"The material for the exhaust system used by most auto manufacturers is 409 Stainless Steel. Unlike the stainless steel used in kitchen sinks for example, 409 Stainless will display some surface rust when receiving heat and/or exposure to chemicals, such as those found in salty water. This surface rust does not penetrate through to the inside of the exhaust system. Thus, there is no durability concern.

There is nothing that moves the exhaust through one muffler and not the other to cause an imbalance. The exhaust comes down a single, center pipe, and then splits in a "Y" to the two muffler branches at the back. There might be a small difference in the length of the branches, but that wouldn't lead to a lot more exhaust through, and water under, one than the other.

It's normal for water to drip from the exhausts when the car is first started and for the first few minutes. Water is a by-product of an efficient combustion process, and is carried out with the exhaust as vapor, but then condenses and drips down from the outlets at the rear. It's more apparent when the outside temperatures are low and the exhaust system isn't hot. But once everything warms up, the vapor comes out the rear at a higher temperature and dissipates in the air instead of condensing.

Having two mufflers reduces the overall exhaust system back-pressure that all mufflers introduce (although this is accounted for in the design of the car). Other than that, I don't see why a single system couldn't work -- some owners have added/substituted various hi-flow systems, albeit for different reasons. But I think the cost etc., might not be worth it. While I haven't been keeping track over the past six years I've been here, I don't recall any number of reports of the mufflers themselves failing due to rust. If anything, it's the flanges or donut gaskets at the front, between the catalytic converter and single pipe going toward the rear, that seem to fail, and that is on earlier generations, at least as far as I remember.
Not to mention that Subaru was trying all sorts of tricks to help address heat vs cooling issues to make the cooling system more robust. The duel exhaust out the back helps carry exhaust heat away in a more efficient manner vs a single tail pipe.
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