What you got I'm sure is the proper design that will work just fine. To control a fan the mobo header does NOT need, nor does it use, the speed of the fan. Fan speed is used for only two purposes: info and amusement value to YOU the user, and monitoring for fan failure if the speed signal stops. Neither is involved in fan control.
Any fan header can deal with a speed signal coming in from only ONE fan. The signal, carried on Pin #3, is a series of two pulses per revolution. If two or more such pulse series are presented to the counting circuits on the header with the unpredictable and changing differences between them, the speed measurement counter gets terribly confused and give wildly changing values. So any proper Splitter or Hub will send back to its header the speed signal of only ONE or its fans and ignore all the others. That is why only one of the two output arms of your splitter has a pin at position #3. When using such a device, since the mobo cannot measure the speed of ALL the fans, you are responsible from time to time to check that all your fans are working and there are no failed units.