Thanks to everyone who has been trying to help me!
It turns out that setting the program to record 90 minutes later does work. But it records the second program for 2 hours. Not sure why, but I'm not complaining. I never got an error message when I set up the series recording though, it just wouldn't record longer until I changed the end sertting to 90.
Helen try what I did. My settings are the same as yours except for setting it to record 90 minutes after the show ends, unless this will keep something else from recording for you. Are you in the Philadelphia area? I get EastEnders from Lehigh Valley PBS 39.