Right now, it's not possible to merge two trips in Tern. The best way to do this would be to save one of the trips as a trip template by clicking the action button on the itinerary you'd like to merge with another trip and clicking "Save itinerary as template".

Then, navigate to the trip you want to merge it with. Drag in the "Itinerary Library item and select the itinerary you saved in the previous step.

That will add the itinerary to the board and effectively merge the two trips. Note that other information about the trip (e.g. traveler information, etc) won't come over as a part of this process.