This is correct behaviour. If using both outputs of the adapter, the maximum resolution will be 2048x1152 (if reported by the monitor). See this article:
http://www.displaylink.com/support/ticket.php?id=329
Quote:
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If using the DL-3900, which supports dual outputs, resolutions above 2048x1152 will be unavailable if a second display is connected. If wanting to use the DL-3900 with resolutions above 2048x1152, disconnect the second display.
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You will need a 2nd USB to DisplayPort adapter to support dual 2560x1600.
Wim