[exim] Combining server_condition

Hello,
i have this working server_condition in the PLAIN
authenticator setup:

auth_plain:
         driver = plaintext
         public_name = PLAIN
         server_prompts = :
         server_condition = ${if eq {} {${lookup 
mysql{MYSQL_AUTH_PLAIN}}} {no}{yes}}
         server_set_id = $2

Now i want to add another mysql lookup, if one or the other
succeeds. In the last hours i tried to combine the 2 conditions
into one big condition, but i don't get it right.

My last attempt was:

         server_condition = ${if or{\
             {{${lookup mysql {MYSQL_AUTH_GWUSERPLAIN}}} {no}{yes}}\
             {{${lookup mysql {MYSQL_AUTH_PLAIN}}} {no}{yes}}\
         }}

, but this doens't give the expected result.
Did i i make an error in combining the conditions ?

Thank you.
Torsten