Two FT-MP RF output were introduced to SixPak A/B input.
Then each band output were went to each amps through DuneStar 
mono band BPF.

I used number of Brands of BPF so far, however this DuneStar 
BPF must be the #1. My 21/28MHz antennas were installed on 
same tower and only separated 2m- however I had no interferences 
on both bands. This was never experienced so far. So I could 
enjoy 2Radios with full KW.
I strongly recommend you to use DuneStar.