A thank you note that comes from the heart is the best kind, but heartfelt meaning doesn't preclude manners. Thank you notes must be sent in a timely manner -- within a week or two if at all possible -- and each person that contributed to the gift must be personally thanked with a note. This means that if two people went in together on one gift, they must both must be thanked.
Number of Notes
Thanking two people for one gift does not necessarily mean you have to send two separate thank you notes. It all depends on the relationship of the two people who sent you the gift. If they are friends or coworkers who live in separate locations, they each should receive a separate thank you note. If they are a couple who lives together, you can send one note as long as you include both of their names in the note and on the address label. Specify the gift, thank the givers for their thoughtfulness, and close out the thank you note by mentioning how much you appreciate both of them.