ok I watched closer today collecting Cobalt/BG/key master
I collected Cobalt then BG. A 2x with the timer showed to the right (on PC) and the BG 2x showed
I got 4 and 8 on the 2 collections
see image
Now, if you did BG then cobalt and it hit, would cobalt do 4x? I don't think so because the 2x is a bonus while it is active after the fact. you would get 2x the 1 FP. I did this yesterday but didn't watch every little detail, I was just excited to get the 8x on 1 of the key master collection. I was so excited, I forgot to collect 2X on my druid hut, it was on 147, so went to 148 when collected but forgot the BG. Didn't get it today with BG, 149, tomorrow, it is off the BG watchlist, finally!!!
edit - forgot image