Click Here to read about a machlokes about how the luchos were written
The Rama of Panno (Asara Maamaros: Chikur HaDin II, ch.2) offers a fascinating explanation, which reconciles all four opinions in the Gemara. He writes that the Luchos were six tefachim wide by six tefachim high, and three tefachim thick. Thus, on the front and back sides of each Luach, there was a surface of six by six tefachim on which to write all Ten Commandments. However, on the sides, top and bottom of the Luchos, the surface was only three by six tefachim. Only five Commandments were written on the sides, top and bottom. It comes out that each Luach had Ten Commandments on front, ten on back, five on the top, bottom, right and left sides, for a total of forty commandments on each Luach. When Moshe Rabbeinu descended from Har Sinai, he held the Luchos one in front of the other. The writing was visible only from the sides, where five Commandments were seen on each – hence R’ Chanina ben Gamliel’s opinion. Later, he separated them and revealed that on the front of each were all Ten Commandments – the opinion of the Chachomim. When he held them up for Bnei Yisroel to see, the Ten Commandments could be seen on both the front and back – R’ Shimon’s opinion. When he placed the Luchos before them to investigate more thoroughly, they saw that five Commandments were written on each side, a total of forty commandments on each – the opinion of R’ Simai. Thus, all the opinions in our Gemara are correct; each referred to a different part of the Luchos.
So, there is no machlokes.