Originally Posted by
wind-blownmind
My question is this: is there an actual rule that says First and Economy may not be combined in this way, or is it simply a matter of whoever designed the public interface thinking nobody would ever want to buy something like that?
The website was probably designed for the latter reason.
But you may want to check whether you can actually combine such fares on your route. On a quick play with
ITA, it looks like for NYC, LAX and SIN (chosen at random for testing) you can combine First fares with WT+ fares but not WT fares; but for JNB you can combine First fares with very low (O class) economy.
If you discover that you can only do First (A class) with WT+ (T class) on your route, you could save yourself a lot of phone time by using the existing online booking interface, given the current pressure on call centres.