Can you tell us how you have the two TelePresence Servers setup within Conductor, what does your service preferences and bridge pools look like etc?
If you want VTS2 only to be used after VTS1 is full, you can create two bridge pools, one for each TelePresemce Server, and add them into a single service preference. The only downside, is that the first bridge will always be used if there are any available resources, even if said resources aren't enough to host the next conference.