I have gone through some similar considerations and could only come up with 1 viable approach, which would be to place your XSJS server side data access layer on both HANA XS instances.
Then from your front end client side JS/?/* code you would make requests to both XSJS data sources (from your HTTP Requests/AJAX/etc.).
Another option with a similar type of process would be to have a XSOData service on both machines for the applicable data set and access them from either HANA or XS instance.
While the concept of HANA + XS is be tightly consolidated and integrated for performance/simplicity benefits, it would be great to have the option to configure our "connections" to solve these types of challenges.