compound subjects

If I were to say: Thank you for your participation in Company A and Company B's "A Rejuvenated America" breakfast -- is only one apostrophe after Company B grammatically correct? Or should it read: Thank you for your participation in Company A's and Company B's "A Rejuvenated America" breakfast? I've been told since it's ONE EVENT co-sponsored by two organizations it is OK to just use only one apostrophe "s" at the end of the two co-sponsors. I, however, think both companies should have apostrophe "s".