This is a partial-temporary solution. Basically, if the frequencies are on different channel numbers then it is possible to "set" the particular wifi band (worked on OSX Mavericks). steps: 1. Find the channel numbers of 5 GHz and 2.4 Ghz. Are they different? if yes proceed to step 2. 2. Set 5 GHz channel number using airport command.
Detailed instructions:
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Open Wireless Diagnostics.app (it is located in /System/Library/CoreServices/Applications).
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In the application menu select Window->Utilities (or press cmd+2).Utilities window should appear.