Isn't just the US and Australia who film in Fiji and pretty sure they have more than one "set" too, although not so sure they film simultaneously. You'd think if they did they'd find a way to film the two CBS seasons at the same time so the crew it out there for half the time.US Survivor just finished filming season 44, so if they are filming in August, then Fiji should be free. Interesting decision to go back to Samoa.
Indeed if they did that and could work some crossover twist in that would be pretty cool - perhaps a Time Travelling Tribal where someone voted out in the fall season then appears in the Spring season at around the same point, and vice versa seeing someone suddenly appear from the future season in the fall season.