Thank you all for your replies.
Just to clarify ... I hadn't bought the tickets yet ... only researching.
I think SIA excess baggage charge would be about US$22 per kg (in my case).
My choice, I thought, was to pay about US$450 in excess baggage or buy an extra ticket(including its luggage allowance) for about US$480. You can see how having that extra seat room would have been the more attractive proposition.
There must be a provision for buying 2 seats in the one name ... what about obese passengers, who are forced to?
However, I think AVMAN has hit the nail on the head ... baggage must be per person, not seat. Is that fair? I don't think so ... but that's life.
I have since bought a ticket on a LCC carrier, including the excess baggage charge, including an exit row charge ... for lower than the SIA single seat charge... and I think I've got a better than 70/30 chance of getting to my destination.