If you are looking at something like HANA Live from multiple systems then check following
How to Realize Cross System Reporting Using SAP HANA Live Content
But if you are really looking at multiple HANA systems and remote connectivity options then may be links below helps out. There are bunch of articles on Cross-System reporting/SDA so may be you can do some research.
I have not tried this so can't give real hands-on feedback
SAP NetWeaver BW on SAP HANA & SAP HANA Smart Data Access
Smart Data Access: Connecting multiple SAP HANA... | SAP HANA