Can I run 100A service from a 200A panel in a garage to a house with an existing 60A service?
National Electrical Code 2014
Chapter 2 Wiring and Protection
Article 230 Services
I. General
230.2 Number of Services. A building or other structure
served shall be supplied by only one service...

If you live in an area covered by NEC, you're only option is to upgrade the service to the house.