In getting healing help from trees, can one enlist multiple trees in the effort, and would that always be an advantage over going to a single tree?

There is strength in numbers; there is additional healing with additional healers, so this would be advantageous to adopt as a general practice. That is not to say one cannot receive significant help from a single specimen, especially if there is a long-standing relationship and a kind of energetic bond as with a special friend or a favorite healer who you have come to know and love—that will add strength of purpose and intention to the task at hand and add to the benefits. But we are talking here of a mature long-standing relationship cultivated over a considerable period of time, so starting out a group approach would be superior.