Should I just not put any number or do I somehow have to get a 2nd number? Am I able to join them once my points post?
The last time I did it, I left the FFN number blank and a new CO OnePass number got assigned to me. The bonus points earned on that CC went to that new FFN. After the bonuses posted, I called CO and had my OnePass accounts merged.
When I applied for my most recent UA card (~3 months ago?), I tried the same method. By some magic, my new UA CC got linked to my current UA FFN.
Luckily I still got all of the bonuses even though the card was linked to a UA FFN that previously received the signup bonuses.
Regardless, I think the consensus is to create a new FFN and apply for the CO card using that FFN.