When you backup portal site collection from Farm1 and try to restore it not as a portal site collection on Farm2, but as a site collection within a managed path, I'm afraid it not possible.
Even after several tries to do it, all I was getting was overwritten portal site collection on the farm that I tried to restore it to. So before you attempt to restore PORTAL Site Coll. into managed path Site Coll. be careful as this action most likely will overwrite your existing Portal Site collection.
And if you found a way to do it without overwriting Portal Site Collection on the existing farm, leave a comment and let me know how.