Quite often during a side by side upgrade I do have the 1 PC being managed by both the old and new servers during testing. As long as both servers have the certificate to match the client then they are able to talk to this client and sent it tasks.
By default the client will send inventory etc to one server but you can specify another in the command line parameters for most of the LANDesk executables to make it report into another core.
What functionality do you require eg SW deployment, inventory etc? This will help narrow down what you need to do to make this work.