The gap that is between the section 'Shema' and the section 'V'hayah im shamo'a' is a setumah (a closed section) and we are accustomed to begin line 7 with 'vhayah im shamoa.  One should write a mezuzah in one column but if one wrote it in two or in three columns it is nonetheless valid. However if one wrote it on two skins even if they were sewn together it is invalid.