I'm trying to implement an sound card-based OFDM modem. A paper says:

...A serial to parallel converter is applied and the IFFT operation is performed on the parallel complex data.

Say I have a stream $S$ of complex numbers, then after serial-to-parallel (S/P) conversion, it becomes $S_1$ and $S_2$. But as far as I'm concerned, OFDM requires the sub-carriers to be orthogonal to each other. So can I just perform IFFT twice directly on the streams without defining the frequencies of sub-carriers? I'm quite puzzled. Any help on this will be greatly appreciated. Thanks.