It's not possible to connect a 5Amp + a 15Amp circuit together to get a 20Amp circuit. It's two 5's, two 15's. The PDM60 can also be programmed to do two, 10Amp circuits. Read the thorough description that we have here
I have wired genuine Stebel horns to a 15Amp circuit on a PDM60 and it's been fine, so long as the voltage when the bike is running is up to snuff. The Chinese copies of the Stebel's seem to draw more current the the real McCoy, and could be an issue on a 15Amp PDM60 circuit.
I'm not sure which Krista harness you have - there have been several revisions/improvements over the last couple of years. I can help you with sorting out the wiring and cleaning things up, if you need.
With regards to the BMW horns, the factory horn conenctor always has ground present on the BROWN wire and +12 volts is switched to the horn on whatever the other color wire is. You definitely don't want to connect the PDM60 BLUE wire to trigger from the BMW horn lead.
BTW, we have a wiring kit
that makes the PDM60 install much cleaner.