Actually, I don't know of any manufacturer who with only one radio can transmit and use two different non-adjacent channels.
I don't think that lack of support for 20+20 channels is unavoidable limitation of wireless chips. It's rather lack of standardization (pretty similar to support for channels narrower than 20MHz) which then caused lack of implementation of such channel configuration. Proof that it is possibke to implement non-adjacent channel configuration is support for 80+80 channels (indeed not every device supporting 160MHz channels supports 80+80).