you guys were right, I gave SIA a call, and they did confirm that for all the flights, (eg the 8 or so per day) each flight only had 1 x saver and 1x advantage seat, so I booked 2 separate, 1 x saver and 1x advantage, all very easy,
admittedly i have been a bit reluctant to call them directly as my past exeperiences i got different answers each time,
