MongoDB text index and text search
So, no need to test as it turns out, I just remembered the key deciding limitations.  You can only create one text index on a collection (reference here), so you have no real choice.  Additionally, MongoDB can only use one index at a time to satisfy a query (until index intersection is introduced in 2.6).

Hence, the only workable option is to create the compound index as you outlined, on both fields as a single index:

db.articles.ensureIndex(
                     {
                       title: "text",
                       abstract: "text",
                     },
                     {
                       weights: {
                                  title: 1,
                                  abstract:1,
                                },
                       name: "TextIndex"
                     }
                   )


The same remains true for 3 or 4 fields, you would first have to drop the existing index, then create the new index and include 3/4/other.