Sounds like you have 2 situations, open house and showing.

For the open house, if you are leaving, can you take her? If you are staying, can you keep her in the car, with the windows just a crack for air?

Showings are more difficult. I take it that you leave the house in the morning for work, and the cats are both in the house as usual. Then during the day with no advance warning for you, some realtor shows up with a potential who wants to see it. That is the more difficult situation, as you aren't even there. For that, the best thing would be if she can stay with a family member or friend.

Microchipping will help some, but that is't fool proof either.