I'm expecting that having gotten the Plus bonus will not prevent your getting the Premier bonus. My belief is based on reports about other programs for which different varieties of cards are offered. However when you login to Chase website your card is not shown as Plus or Premier. It's just a RR Visa card, and you can't tell even from the statements which kind it is. So, yea, it's a guess that you'd get the second bonus. I think your chances are very good, but it's not a sure thing.
You're right, quite a few have posted in the master 50k RR thread that it is possible to get the sign up bonuses for all four cards. That's how I'll be holding onto my CP starting next year.