Currently you can only copy items on a Quote to the current Quote you are on. It would be ideal to be able to copy these to another Quote (similar to how you can copy items on a Change Order to another Change Order). Currently the only way to do something remotely close to this is create a Package of the items you want to copy and then adding that Package, but this has some limitations and caveats and it would be much better to copy the selected item(s) to another quote as-is.
You can also use the copy location and system to move items from one quote to another but you don't always want to add locations/systems to copy items.