From the T&Cs:
"This one-time bonus offer is only valid for first-time United MileagePlus Explorer cardmembers with new accounts. United MileagePlus Explorer cardmembers will only receive one 50,000 bonus miles award if they apply for both the United Explorer Card and the Continental OnePass Plus Card after 7/19/2011."
Who will have a M+ Explorer Card before anyway?
Beside, it is the T&Cs for OP+:
"This one-time bonus offer is valid only for first-time Continental Airlines personal cardmembers with new accounts. Previous and existing Continental Airlines personal cardmembers/accounts are not eligible for this bonus offer."
Based on this, it has cleared all existing M+ cardmembers to get the bonus again.