I have a customer that has two receptionists and they want to have one mailbox shared between their phones. They told me that they had had this setup in the past and the one mailbox would trip the msg waiting lights on both their phones.

Is this possible?

Yes
From admin go to the properties of that mailbox and add up to 2 more DN's.

Note that you can only do this if they share 1 mailbox and not have any other mailbox tagged to their sets.

I gave you bad info. Sorry. It's not a Call Pilot. It's a Flash.

Then you might want to utilize Answer DN keys and Off Prem Notify.....I dont see how they had it light 2 sets with a Flash.