Out of several commentaries, the only one I can find directly addressing the possibile symbolism of the horns of the second beast is in Warren W Wiersbe's commentary on Revelation:
"The image of the horns (13:11) suggests that the false prophet [the second beast] has authority but the absense of a crown indicates that his authority is not political."
While numerology plays a significant role in Revelation, I cannot find any substantive, consistent use of the number 2 that would directly link the horns to the witnesses.
There certainly is symbolism in the uses of the lamb and dragon images (perhaps suggesting that he may try to come off as the Lamb of God but in actuality is the dragon, "the ancient serpent who is the devil and Satan") and to add the detail of two horns is certainly specific enough that it may have meaning as well. However, since the beast from the land is described in detail two chapters after the foretelling of the death of the witnesses at the hand of the beast, I am not sure there is a correlation between the horns and the witnesses.