I'm of the opinion that both brothers could theoretically get the eyes. I believe that the eye surgeries of the manga consist of simply taking the eye out of one host and implanting it into the socket of another, similar to Kakashi receiving Obito's eye and Ao receiving the hyuuga byakugan.
However, coincidentally in both cases of a brother taking the others eye, one of the brothers was always dead (Izuna and Itachi.) Itachi did claim that unlocking the the mangekyou in general was not something everyone in the clan was capable of doing, and the odds of both siblings in a family unlocking the eyes must have been equally rare. Perhaps Shisui had a brother that died before being able to unlock the eyes?
If Itachi was not dying/ hoping to redeem himself through death, it seems logical to me that both he and Sasuke could have received eternal mangekyou sharingan. However it did take Itachi's death for Sasuke to unlock his mangekyou in the first place, so we will never know.