I've not been to Antarctica but have been to Africa and to the Galapagos where two bodies are an absolute must (not many camera stores in these places). In addition to the basic reliability issue, you can put different lenses on each and not change lenses as often. Sea water is evil stuff when it gets near a camera or lens. I'd not want to get into a Zodiac with both my camera bodies. The A9 will certainly be good for wildlife (penguins, seals, ? whales-probably not polar bears :-)) and an RIV or A1 will easily take care of the rest. You may want to read Lloyd Chambers take on the A1. He says, "Sony A1 image quality overall seems to be the best ever seen in a Sony camera. My impression is that of improved tonal and color rendering over the Sony A7R IV." If you don't need the speed, or if you don't plan on making the A1 your primary camera later, the RIV is hard to beat for image quality. I suspect I'd not be able to tell the difference, especially when the price difference is factored in, eg almost 2 A7RIV's for the price of an A1.
Whatever you do don't take more than one system; it will make you crazy. I did that on a southwest workshop years ago and spent way too much time deciding which to use and my photography suffered. The A1 apparently has lossless compression, a wonderful sounding thing to me whose RIV landscape files, when all done, are usually over 1GB. Bring an extra battery charger as well as additional batteries and lots of cards. Even with uncompressed RAW, I get about 1,000 images on a 128GB card. If you take more than that in a day, I don't want to be your editor :-) While it's likely someone will have a Sony camera and its charger, any digital camera is a doorstop without a battery. Larry's idea of checking in with the tour operators about what to bring is a good one. Clearly you need somplace to download and backup your files each day, a laptop or equivalent with a few backup drives is great. I use Samsung T5 SSD's 1 and 2TB when I travel.
I'm envious of most of your trip except for the Drake Passage. I get seasick easily. But goods luck, have fun, be safe and share upon your return.