I am reviving this thread in order to ask if the situation is the same as five years ago, when the last post was written.
I have an upcoming trip LIS-IAD on TAP, then IAD-LAX (same day) on United; two separate tickets and therefore separate PNR's. The separate tix were necessary because one is paid and for the other I used miles. Will I be able to check a bag all the way through to LAX from LIS?

Thanks to anyone who can give an update on this question.