Only way is to switch off the Bluetooth on the 2nd phone. It is ALWAYS the newest phone to be paired (if present in the car) that will connect.

So, if you have 4 phones paired, and have phone no. 1 and no.3 in the car, phone no. 3 will pair etc.... It's a real PITA. My Mrs. Toyota has a facility on the nav/computer screen to manually switch ... the BMW isn't that sophisticated.