Under-leas is already tapped. A Balrog company enters this site, deals with automatic-attacks and play Liquid Fire (playable on tapped sites).
A second item can now be played.
The Balrog player want to play Aiglos (playable at an Under-deeps Shadow-hold).
What happens ?
1) Aiglos is playable as a second item
2) Aiglos is not playable because the site is already tapped
DM rules:
"Normally, when you successfully play an item, faction, ally, or information at a site, the site card is tapped and one additional character may tap to play a minor item. At an Under-deeps site, you can play any item playable at the site instead of this minor item."
When site card is untapped You can play Aiglos as first or second item, but if site card was already tapped (for example was tapped last turn) only items playable "at tapped sites" (like Liquid Fire) can be played.