TPS61196: Multiple driver channels to the same LED string - different number of LED's per string

Part Number:TPS61196

My application has a 13.6V input and I need to drive three LED strings at 150mA each. Two of them have 6 LED's, and one has 8 LED's, with 3.2V Vf max each. If I use individual channels, everything works fine. However, with the intent of better dissipate heat in the device, I wanted to use two channels per string. Here is my configuration:

With this configuration, only the string with the largest number of LED's works, in this case the 8-LED fixture. If for instance I modify the diagram above and use 4 LED's where the 8-LED string was, then the two banks of 6 LED's turn on, and the new 4-LED bank stays off. 

Is this the way the part is supposed to work? Any workarounds?

Thank you!