would it be better to use another SQL action slot if I want to subscribe a user to 2 or more newsletters at the same time or can I alter the code to do this in one action?

You can do that in one query:

[code:1]INSERT IGNORE INTO `#__acymailing_listsub` ( `listid`, `subid`, `subdate`, `status` )
VALUES ( <listid1>, ( SELECT `subid` FROM `#__acymailing_subscriber` WHERE `userid` = [user_id] ), UNIX_TIMESTAMP( NOW() ), 1 ),
( <listid2>, ( SELECT `subid` FROM `#__acymailing_subscriber` WHERE `userid` = [user_id] ), UNIX_TIMESTAMP( NOW() ), 1 ),
( <listid3>, ( SELECT `subid` FROM `#__acymailing_subscriber` WHERE `userid` = [user_id] ), UNIX_TIMESTAMP( NOW() ), 1 )
[/code:1]

thanks - what about multiple deletions upon deactivation?

About the deletion:

[code:1]DELETE FROM `#__acymailing_listsub` WHERE ( (`listid` IN (<listid1>,<listid2>,<listid3>«»)) AND (`status` = 1) AND (`subid` = (SELECT `subid` FROM `#__acymailing_subscriber` WHERE `userid` = [user_id] ) ) )[/code:1]

That's great!