Replicates have to be in the same database. Think of it like this: if you have an alias in a Finder folder that links to an original on a removable USB drive, and you remove that drive, then Finder will not locate the original and you will get a “not found” error. Similar thing with replicates – they are like aliases pointing to the original. If the original is offline (the source database is closed) the replicate would fail. Thus DT doesn’t give you a chance to do it and fail.
Duplicates are just that - copies that are no longer linked to the original. Thus, they can be anywhere.
The advantage of a replicate is that when the orignal changes then all copies changes (or if you change a copy you change the original). If you really need a replicant, here’s a workaround: keep the original in a Finder folder and index that original into all databases where you want to access the original.