The short answer is that you can't. Gazell (pairing) was not designed to dynamically switch between host and device mode. If you have such a requirement, you should base your application off the ESB library, then build synchronization, hopping and pairing requirements around it.
No, that is not possible. Though this is often more of a theoretical problem than an actual problem. Typically the time on air is very short, which reduce the risk of collision. Also the host and device pair communicating is typically closer together than other host and device pairs, in such a case the signal strength of the interfering host and device pair is typically attenuated, this still allow communication at the same time.
