A friend of mine is flying El Al next month from Chicago to Tel Aviv and he wanted to use his Matmid points to get a second suitcase. The flight is actually an AA codeshare flight from Chicago to London, and then the flight from London to TLV is LY metal.

So on the Matmid website when we went to use the suitcase award, it says:

"Not valid on code share flights" for the Chicago to London leg,
and then right underneath it has a checkbox that we can select for 200 Points for the London to Tel Aviv flight.

So is it possible to do a suitcase award in this case? I dont see what would be the point of letting us select the award only for the London to Tel Aviv leg when the first flight originates in Chicago.
What would happen if we selected it and showed up in Chicago and tried to check in the second bag at American?

To clarify the first part - it's an LY codeshare on AA Metal from ORD-LHR, and then an LY flight from LHR-TLV. The ticket was purchased directly from El Al.

Since ORD-LHR is the longest segment in the itinerary, and is operated by AA, then AA baggage policy applies...