Cat bed/nest

I said in another thread I had got a small cat bed for Monty. He loves going in it, so does Mitzi and she has been getting jealous if Monty is in it. I now have two new trolleys (a long story) and today Mitzi was on her stand on one of them. On the other I had placed the cat bed and put Monty in it. That trolley is smaller so the cat bed was lower down than Mitzi's stand. Mitzi went to the end of her stand and was leaning over trying to nibble the edge of the cat bed. I told her not to do that so she resorted to furiously chewing a fabric wrist rattle that is on her stand and which she is allowed to chew. After a while she turned her attention back to the cat bed and she stretched herself so that she could climb down onto the cat bed. Once there she gave Monty a kiss and then started preening. They were very close to each other, something they never do in the cage. 

I don't think I can allow this to happen again because I have a feeling Mitzi might see it as a suitable nesting site.

Jane