The John Rylands Library in Manchester has a portrait said to show both Christopher Marlowe and William Shakespeare.

Anonymous: Young man. 1588. John Rylands Library. Manchester CC0
The Grafton portrait is painted in oil on an oak panel measuring 17.25 x 15.5 inches (44.5 x 38.5 cm). In the upper left corner is the inscription "AE SVAE [aetatis suae] 24" in the right corner is the numerical sequence "1∙5∙8∙8". Research confirms that the work dates from the Elizabethan era and was made in England.1
Unlike the Cambridge portrait, the provenance of this painting is quite well documented. It probably hung in Grafton House, which was destroyed in 1643 during the English Civil War. Andrew Smith, one of the tenants at Grafton Regis (Northamptonshire), brought it into his possession and passed it on within the family until it came to a Ms Ludgate who sold it to Thomas Kay who bequeathed it to the John Rylands Library in Manchester in 1914. The assumption, expressed in the Manchester Guardian as early as 1907, that this was a picture of Shakespeare made during his lifetime,2 also reappears in more recent Shakespeare literature.3
The John Rylands Library does not claim that the man in the painting is William Shakespeare or Christopher Marlowe. Professional portrait painters are said to have independently concluded that the Cambridge portrait and the Grafton portrait show one and the same person painted by two different artists, each in a different style.4 Here, not only the art but also the similarity is probably in the eye of the beholder.