UA and AA still interline, but you also require the fares to allow combination and it seems like an increasing number of domestic fares on both sides (and perhaps all fares for AA) require combination with only own carrier fares (UA is UA/AC).
e.g. Category 10 on SFO/ORD:
Code:
2-COMPONENT CIRCLE TRIPS
FARES MAY BE COMBINED ON A HALF ROUND TRIP BASIS
-TO FORM 2-COMPONENT CIRCLE TRIPS.
PROVIDED -
COMBINATIONS ARE WITH ANY FARE FOR CARRIER UA/AC IN ANY
RULE AND TARIFF.