According to the OSGI specification, a fragment bundle in OSGI can have multiple hosts. But in apache karaf it does not seem to work.
I have tried the following.
Fragment-Host: default_host;bundle-version=1.0.0,Second_host;bundle-version=1.0.0
You are right that a fragment can have multiple hosts according to the specification. But it seems to be limited to different versions of bundles with the same symbolic name:
The grammar for Fragment-Host is defined as
Fragment-Host ::= bundle-description bundle-description ::= symbolic-name ( ';' parameter )*
and by using a version range, you technically define multiple host:
bundle-version - The version range to select the host bundle. If a range is used, then the fragment can attach to multiple hosts.
It doesn't seem to be possible to attach a fragment to host bundles with different names like in your example.