[Koha] 'connect' various koha instances

[mourik jan heupink]

Hi all,

Could anyone tell me what our options are, if we would like to integrate 
a few (two, three, four) different koha installations that are currently 
on completely seperated networks/continents.

As in:
- search results on kohaX also gives results found in kohaY and kohaZ
(plus vice versa etc)

Note: I don't mean actually merging the different koha's into one single 
installation, just integrating search results.

(this is (probably?) very easy, I just don't know where to look for)

Thanks,
Mourik Jan