If only you could see what I have seen with your eyes

Solution 1:

The 'person' that says that line is "Roy Batty", he is a replicant, a robot that looks like a human, called a Nexus-6

The old man he was talking is "Hannibal Chew", he works for the company that makes replicants making artificial eyes.

When they meet, Batty wants to know about incept dates (a built in mechanism to prevent the replicants living for a long time).

Chew doesn't know anything about incept dates and says to Batty "You Nexus, huh? I design your eyes." [1]

Batty holds one of the artificial eyes (not his own or one Chew's real eyes) and tells the old man "Chew, if only you could see what I've seen with your eyes" [2]

So he means, if only Chew could see what Batty has seen by using one of Chew's artificial eyes, which in almost all senses could not possibly be a common idiom in English at the moment (but it sounds very like one).

[1] Hong, James (as Hannibal Chew), perf. Blade Runner. Dir. Ridley Scott, Writ. Hampton Francher, David Webb Peoples, and Philip K. Dick. Warner Bros (USA), 1982. Film. 9 Jul 2014. http://bladerunnerthemovie.warnerbros.com/.

[2] Hauer, Rutger (as Roy Batty), perf. Blade Runner. Dir. Ridley Scott, Writ. Hampton Francher, David Webb Peoples, and Philip K. Dick. Warner Bros (USA), 1982. Film. 9 Jul 2014. http://bladerunnerthemovie.warnerbros.com/.