The above posts clearly explain the diffrence between XA and local TX.
For your other question ie merging of two ds files
1)do you want to add two defn's in one ds file
yes you can certainly do it as explained in the above post
2) your endpoint is same for both defn's and just want to use one common def'n for all txs
Just use the XA datasource it can serve both the purpose with an overhead of negligible increased response time