I have booked a reservation with Iberia. It is ticketed and paid for, and contains flights with El Al, BA, AA, and Iberia. The flights on IB and BA can be changed on the BA or IB sites with the booking reference or linked to my Avios. I would like to view the reservation on the El Al and AA websites to change the seating on those flights. The IB reference does not work on those sites.
Using Check My Trip.
You can use Check My Trip from Amadeus to view the record locators for other airlines, if the prime PNR record is on Amadeus. In this case Iberia uses Amadeus so (unless a travel agent issued the ticket) you can indeed use the Iberia PNR.
The website is a bit clunky. Click on the red "Login" button which takes you to the main page (you don't need to have an account). Enter your Iberia booking reference and last name. This brings up the master record.
To find the El Al locator, scroll down to an El Al flight and click View Details.
You may find that there are several record locators for the same airline, depending on the complexity of your itinerary.
Using My Flights.
If you have an iPhone or iPad the My Flights app is an essential companion for the frequent traveller. It will show a detailed breakdown of any Amadeus PNR and will alert you if the airline makes any change to your PNRs. There is also a web version at http://www.myflightsapp.com/. However, you do need to sign up.