Pardon me if I'm telling something you already know.
I'm going to be purchasing an Eclipse this spring, so I don't about peculiarities associated with Halcyon BC's yet, but my SeaQuest had a single strap, and was prone to slippage with a single HP100 Steel (Genisis w/ gloss finish).
I don't bother soaking the bands. Instead I feed the band thru the first loop of cam lock. Next I hold the tank & BC between my knees with one hand grabing the band as it exist the cam.
Next I twist my wrist and while rocking the cam lock back and forth, with a little snap on the lock works well (the delayed memory of the band works to your advantage here).
Leaning on the cam lock crates enough friction to allow me to change my grip and continue sinching more slack out of the band. Using this method, I was able to eliminate all slippage.
All Halcyon's have two straps don't they?