We don't currently make any
Though devices such as the DAP series, or even other manufacturers adapters (a majority of wireless clients are built into notebooks in my experience) are still capable of making that a worthwhile feature.
Though at the end of the day, I think that if the feature is useful to you you will gravitate towards it, if not, well then you won't. We only sell what customers buy. Considering the number of customers in this very forum searching like nobody's business for an DIR-855 I think that someone must have a desire for the functionality.
Now if this whole thread is a request for a desktop adapter using the PCI, PCI-X, PCI-Express, Firewire, RS-232, Short, ISA, Micro Channel, PC Card, or Express Card buses that supports concurrent 2.4 Ghz and 5.8 Ghz draft N, then consider it duly noted.
Now if this whole thread is a request for a desktop adapter that supports concurrent 2.4 Ghz and 5.8 Ghz draft N, and for it to load balance over the 2 radios of a DIR-855, then I can still take that as a feature request, but I can also tell you its flat impossible.
If I missed your point entirely perhaps you should try to elucidate it for me.