just start only 8.1.7. listener,
and the second database can be configured on the SID description of the listener.ora file:

SID_LIST_LISTENER =
(SID_LIST =
(SID_DESC =
(SID_NAME = PLSExtProc)
(ORACLE_HOME = D:\home for 8.1.7)
(PROGRAM = extproc)
)
(SID_DESC =
(GLOBAL_DBNAME = db_8.1.7)
(ORACLE_HOME = D:\home_8.1.7)
(SID_NAME = sid_8.1.7)
)
(SID_DESC =
(GLOBAL_DBNAME = db_8.6)
(ORACLE_HOME = D:\home_8.6)
(SID_NAME = sid_8.6)
)
)