I don't believe it is cheating from the separation anxiety, because they are not your T. However, I do believe if the anxiety is strong enough, seeing a back up T can take the edge off of the anxiety which may be needed to help someone get through a rough patch while their Therapist is sway.
I have only seen this back up T twice, but I try to see the same one while he is away. There is not a wrong or right way to do this, but if it helps, do it. I am sure your T. does not want you suffering and struggling to deal with the separation anxiety.
These are just my thoughts from my experience as recently as today. Hope it helps. All the best.
T.