The Victron Smart Solar 75/15 allows you to go up to 75 volt down to 12 volt. So you can put them in series, because then it is easier to reach a voltage higher than the battery and you have a bit more charging power.
However, you can put them in parallel as well, that will also work, and keep working when one of them fails. Or at least fails in such a way that it doesnt make contact anymore. If you're not getting any solar power anymore you can disconnect one at a time to figure out which one is still working. So you can easily fix it on the road.
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2011 Overland from Amsterdam to Nepal and back
2018 Overland from Amsterdam to Tokyo via Central Asia and Mongolia
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