It is not easy for new facilitators to be the process guardian and proposer/objecter at the same time but once you get used to the process, it becomes quite easy and natural.
One way to help you is, as Karilen suggested, to make a physical change of space and you can even have markers on the ground. You could also do that with different hats : Facilitator hat, Proposer hat.
I often see new facilitators missing themselves during reaction rounds or saying “no reaction” because it is difficult to handle the different stances: their defined roles and the facilitator role. I think it mainly comes with practice and if you are not comfortable because you are emotionally attached to the tension/topic for instance, you can handle it to someone else.
There are no specific rules around that - at least in the constitution - and if you feel a tension about it and need to define a rule, that's fine too.