With each fan drawing a maximum 1/3 amp, you should be OK using a splitter cable where one yellow wire is not connected to one of the fans.
I prefer to use a front panel fan controller where I can set the fan speeds manually.
Fan controllers differ on how low they will lower fan speeds. Read reviews to be sure that a specific controller will undervolt well.
I'm not familiar with the Akasa AK-CB002 unit, but it sounds like a good way to go, but it might not include the yellow RPM sense wire that would connect to a motherboard fan header to report fan speeds.