According to the MarkLogic docs, the "collection" parameter can appear multiple times in a GET request to the REST Client API; however, the following returns 0 results:
/LATEST/search?q=&collection=GEWN&collection=TSJN
whereas an XQuery specifying both collections on the same database does return the expected results:
count(collection(("GEWN","TSJN")))
=> 90871
Using REST API to query one named collection individually returns expected results also.
Thanks.