This is an old post of mine that demonstrates how two siblings could inherit two different genes. In other words, this is why it is possible for the Uchiha not to have the relevant Senju genes to be the SO6P and vice versa.
Thats an excellent point. Though I'm not really implying that their blood specifically is different. It would be closer to accurate in saying that I am implying that some of their gene's are different. More specifically the genes that matter. Like the Rinnegan gene, and the his chakra genes, etc.http://www.mangafox.com/manga/naruto/v54/c510/11.html
"Uchiha and Senju were once one!"
Elder son was how you say "hybrid" from the day he was born. Same goes for younger son. Basicly, where you are wrong is that you are implying younger and elder son have different blood when infact only thing that is different in them is the powers they inherited from their father.
, I can't think straight right now, hope you understand what im trying to say.
Heres an example if you look at it genetically (I'm no expert on the subject): I have a cousin with blue eyes and her brother has brown eyes. Its the same with their parents, their mother's eyes are brown and their father's is blue. You probably know already that the gene that causes eyes to be Brown is dominant and the gene for blue is recessive. Also everyone's genes come in pairs. In order for my cousin to have blue eyes, she needs to inherit two blue genes. The reason her brother has Brown eyes is because he inherited one brown gene and one blue gene. Brown is dominant and blue is recessive and thats why her brother's eyes are brown. Which means that some of their genes are different. In this case the genes that matter are the genes that determine eyecolor.
Now lets say the Brown gene is the Elder son's Rinnegan and the blue gene is the Younger son's normal eyes' genes. It would mean that the younger son did not inherit any Rinnegan genes.
Edit: Also I think that what was meant by "Uchiha and Senju were once one!" is that the Uchiha bloodline and the Senju bloodline originates from one person. That person being the SO6P. I could be wrong of course.