Originally Posted by
yytleisure
I think the answer to the OPs question is yes and no. You can (possibly UA, definitely other airlines) check in online for later segments, but that does NOT check you in for the first segment. IME.
UA does not allow online check-in if there is an earlier partner segment still on the reservation. OP can check in online once the coupon is lifted -- generally, at some point after the partner boarding pass is scanned, depending on how quickly that information gets passed through to UA.