1) WordPress.com Custom CSS plugin (vis-a-vis The Jetpack plugin). That way edits to the theme on blog 1 stay on blog 1. The actual theme files do not change.
2) Some trickery in the child theme would be needed.
If theme2 is a child of theme1 already, then you must override in theme2 what appears in theme1. Any change to a parent theme will be reflected in all child themes.
SO I offer this.
Install your parent theme, but make both theme1 and theme2 children of the parent theme. Do not network-enable the parent theme.