I’m currently working on some mods to my ST2 for long distance touring and they include a handlebar bag (similar to the Frontloader) from a company named Bedrock that is attached to a Jones HBar handlebar. With a tent in the handlebar bag, several separate accessory pouches and 2 front fork bags I can confirm the steering feel does get altered. Not bad, and not dissimilar to the front weight impact you would experience on a regular touring bike. I’d have to say it feels like the initial stability is a little more “shaky” but once you get rolling it’s so sweet and solid.
I’ve also toured with both my ST2 and a traditional vintage road bike pulling a Radical Design trailer and found it great for hauling way more gear than you need. I’d say if you had to take a second battery and you know the road conditions are amenable to an extra set of wheels the ST2/trailer is a great rig. It gets all it weight for your bike, allows for easy pack/unpack and makes for really slick rig.
For my next tour, a 2-3 month self contained ride from the West Coast to the Midwest on my ST2, I’ve chosen a plan as Ravi suggests of adjusting my schedule to accommodate longer lunch break charging combined with evening charging at campgrounds or perhaps an occasional cold beverage or two at an outlet friendly local watering hole. Using an assist strategy of Level 1 or none on my standard 814w ST2 I’m pretty confident from my fully loaded tests so far that I can consistently do 60-80 miles days on a single charge. Personally the extra weight of a second battery on the bike itself, other than in a trailer, along with additional charge time for 2 batteries just don’t make it worth the effort.