Per Steinberg; you can only have 1 arranger track per project. Thats the hard and fast rule. Assuming you already know that though.
I know you can save a duplicate version of the Arranger Track by selecting to flatten it and simply rename it without making any changes to the arrangement.
That would simply give you another project file with duplicate Arrangement track but all the content referenced in the Arranger track would still be shared within the project. You could also backup your project to a different folder and it would include its own duplicate version of the Arranger track. You could then make modifications to the backup project using a duplicate of the original Arrangement track.
Another possible alternative, although I haven’t personally had need to do it; would be to backup the project to another location (obviously). Open the backup project, empty the project of all media and track content so you essentially have an arranger track and empty project. Then save that as a Project Template with the arranger elements of the Arranger track without any associated media to use.
But like I said, not 100% certain of what it is you’re trying to accomplish by copying an arranger track between 2 different projects; or if I understood the question correctly. Unless the 2 projects have identical track layouts for the number of bars per arrangement segment and etc. I don’t understand the purpose. Sorry