Imagine you have in server C a replicator database with the two following pull replication documents in it:

{
     "_id": "rep_from_A",
     "source":  "http://aserver.com:5984/foo",
     "target":  "foo_a",
     "continuous":  true,
     "_replication_id":  "c0ebe9256695ff083347cbf95f93e280",
     "_replication_state":  "triggered",
     "_replication_state_time":  1297971311
}
{
     "_id": "rep_from_B",
     "source":  "http://bserver.com:5984/foo",
     "target":  "foo_b",
     "continuous":  true,
     "_replication_id":  "231bb3cf9d48314eaa8d48a9170570d1",
     "_replication_state":  "triggered",
     "_replication_state_time":  1297974122
}

Now you would like to have the same pull replications going on in server D, that is, you would like to have server D pull replicating from servers A and B. You have two options:

Both alternatives accomplish exactly the same goal.