TNS-12154 TNS:could not resolve service name
Cause:    The service name specified is not defined correctly in the TNSNAMES.ORA file.
Action:    Make the following checks and correct the error:
Verify that a TNSNAMES.ORA file exists and is in the proper place and accessible. 
See the operating system specific manual for details on the required name and location.
Check to see that the service name exists in one of the TNSNAMES.ORA files and add it if necessary.
Make sure there are no syntax errors anywhere in the file. Particularly look for unmatched parentheses or stray characters. 
Any error in a TNSNAMES.ORA file makes it unusable. 
See the SQL*Net V2 Administrator's Guide. If possible, regenerate the configuration files using the Oracle Network Manager.

TNS address is not resolved on the client, it is resolved on the server that client's SQL*Plus session is connected to.

so, make sure u have the tnsentries
in in database1 and database2 ( ie. tnsnames files of these two servers).

by convention , create db link name same as connect string.
if it is not helping you,
create db link like this
-- note: change SID to SERVICE_NAME accordingly
-- or just copy the entries from tnsnames.ora
create database link test
connect to scott identified by tiger
using '(description=(address=(protocol=TCP) 
     (host=xxx.xxx.x.xx)(port=xxxx))(connect_data=(sid=xxxx)))'; 

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 if any of these is not helping, post the tnsnames.ora, and ur session

as per your posting
[i] USING '/SID/' [/i]
are u usid sid or the connectstring?
you must use the connectstring not the sid

your statement should be somehting like
CREATE DATABASE LINK [b]dblinkname[/b] 
CONNECT TO USER IDENTIFIED BY PASSWD 
USING [b]'/SID/xxx.de' [/b]-- as appearing in tnsnames.ora
                          -- and this tnsnnames.ora should be in server, NOT in your client