Yes, you should disclose that you're in contact with another potential supervisor in that uni if the department is quite close. How about you bring up the idea to be supervised by both of them? You need two supervisors anyway, and if they are both interested in your research proposal and publish together they would make an excellent supervision team.
I wouldn't see it as mass-contacting, just shape it into your awareness of knowing how strong the faculty is and how well you'd fit into that environment if you had two people that you'd love to learn under.