merge 2 medias database (networker 7.1.2)

Use scanner command. Read the manual for it and make sure you understand what it is and what it does. Let's add that you will need around 120 minutes to scan single tape (perhaps 90) and during that period you will have one drive less available (at least).

How can I do that ?
all my clients in the same group -> backup group
and ?
I can't use mmrecov, because db on Server2 will be overwrite I think.

You can't merge two media dbs. However such request exists and hopefully it will be addressed one day in the future. What you can do in the meantime is to scan your tapes from Server1 as that is the only way to populate media db on Server2 with data from Server1.

The command the most usefull in scanner manpage :
scanner -m # Rebuild the media indexes for the volumes read...

but, I can't do this for all media because we have more than 200 cartridges
120minutes*200= !:|

Do you know a faster solution ?

There is no faster solution to achieve what you want (merge two dbs) - scanner is the only option here. However if you go one step before you did get to merge idea (which is consolidate both backup server as one) then you have ask yourself what do you want exactly. For example, if you wish to keep old data for restore purpose you could migrate this somewhere else and keep it down until there is need for restore. Or if restore requirements are frequent move both server to VMWARE box and share infrastructure, etc. But merging 2 media database (without scanner) is impossible.