Originally Posted by
Singapore_Air
It could be that no one at SIA has constructed such a fare and therefore indeed this is considered by SIA as two separate legs that need to be booked as two separate one-way fares.
On ExpertFlyer I can't seem to bring up FRA-SIN fare rules (it indicates, but it does indicate the following (for all Fare basis codes):
Which would suggest that FRA-SFO fares from SQ must be on FRA-JFK then codeshare across to SFO (rather than via SIN). Which makes sense via JFK it is ~6500 miles, via SIN it is ~15000 miles.