Can you have a FaceTime session while also using another VOIP solution like BlueJeans or Skype
No.

While it is possible to have 2 simultaneous active VoIP calls (using different solutions) on your iOS device, it is not possible to be heard at the same time on both of them. As soon as you make the second call, the first one is put on hold.

You can switch between the two, but you can be conversing only over one call at a time.