I have two AM accounts, one in my name and one in DH name. I do most of my collecting on DH account, it was the first one opened and is Onxy. All of that goes into Dream Miles but I do transfer over to Cash as needed for promos. The second account is one I use for the bigger promos and it is all in Cash miles. That one I use to mostly get gift cards from their selection. I have used it to pay for Via Rail when a group of us went away. It saved me $1000 right there. On this second account I rarely get all 5 offers completed but even getting 3 done it puts a few airmiles in the account to use for the gift cards. Two accounts, for me, is a bonus. As I said, I do concentrate on the first one but I don't ignore the second when there is a good offer.
As for the BMO CC, I would get one if only to have when you get offers. I have a PC MC as my secondary CC that I use for card link offers with AMs for my second account. That means I get to do an easy cardlink for any promotions that come up on my second account. With BMO, there has been an occasion or two where they offer a AM bonus if you use the card so it might be handy to have it. Especially if you do open that second account.
That's just how I do it. It's all pretty much a game and we all play differently. So much fun though.
