1. You can use the phone simultaneously with a Bluetooth application. No interference should happen (cf. Dial-Up Networking use case where you have a Bluetooth link between a PC and a phone, and simultaneously a data call/ GPRS connection).
2. Bluetooth application can work in the background. It depends on the Bluetooth implementation of the vendor whether you can have multiple Bluetooth links simultaneously. In Nokia 3650 you cannot have Bluetooth data links simultaneously with a Bluetooth