Rashi

זמורה (lit., AND THEY CUT DOWN FROM THENCE) A BRANCH [AND ONE CLUSTER OF GRAPES] — i.e. a vine branch with a cluster of grapes hanging from it (not as the text might suggest, that they cut a branch and they cut also a cluster of grapes),
וישאהו במוט בשנים AND THEY BARE IT UPON A POLE BETWEEN TWO — From what is implied in the statement “And they bare it upon a pole”, do I not know that it was carried by two men”? why then does it add the word ‎בשנים?‎ It means, not as you assume, “by two men” but “upon two poles”. How was that done? Eight of the spies bore the cluster of grapes. Besides these, one spy took the fig and one the pomegranate, but Joshua and Caleb did not take anything (although all of them were bidden to do so; cf. v. 20), because the very essence of their (the other spies’) intention was only to bring an evil report: “Just as its fruit is extraordinary in size, so is its people extraordinary in size”. — Now if you wish to know how much was the burden of each of them, go and learn from the weight of the stones which they erected at Gilgal; for it is related (Joshua 4:5 and 20) that each man lifted by himself one stone from out of the Jordan on to his shoulder, and set it up at Gilgal; — and our Rabbis ascertained the weight of each stone to be forty Seahs, and it is an accepted fact that a load which a person can by himself lift up on to his shoulder is only the third of the load he can carry if people assist him in lifting it up (Sotah 34b).