I don't think my answer is 100% right, but maybe it will work toward the right answer :)
H2O (or maybe OH)
I have two that are different,
Oxygen and hydrogen
I often grow to be stronger,
Not sure about this one, but I think it has to do with hydrogen bonds.
And when you come into me,
I might be found in the sea.
The sea consist out of water
I have two both the same,
2 times oxygen
One day I might bifurcate,
O2 can bifurcate when it gets extremely heat (I think), and when it attach itself to another molecule. Correct me if i'm wrong!
And the biggest part of me
Can be found in in real life.
We breath oxygen and a big part of the air consist of oxygen
The connection between 1 and 2:
Water and oxygen complement each other