In agreement with what Jock has written, I will add that from the heraldic standpoint of "tartan inheritance" (as opposed to any sort of genetic/DNA inheritance) one would usually assume the named tartan of their nearest Scottish ancestor, whether that was in the paternal or maternal line. So, if you go back two generations on your mother's side before you reach a Scottish ancestor, but have to go back six on your father's side, then you would most likely adopt the tartan that first occurs in the maternal line. It is this practice that has led to the concept of "wearing your mother's tartan" if there is no clan tartan associated with your surname.

Now, that said, because this is a matter of assuming, rather than inheriting, a tartan there is no legal reason why you couldn't adopt a tartan from the paternal line. If you did, you would be taking the position that however many generations ago your male ancestor became, through marriage, a member of Clan X, accepted the Clan X tartan as his, and that that "sense" of being a member of Clan X has passed down, through the generations, to yourself.

Personally, unless one has a "by blood" or "by adoption" connection to a specific clan I would advise against indiscriminately choosing a clan tartan merely on the basis of "Gee, that looks nice". Likewise, in total agreement with Jock, I would suggest that once you determine your Clan affiliation, you stick with it, to the exclusion of all others.

In the Bible it says, "You can not serve two masters and love them both" and in those places where the wearing of tartan has meaning, wearing more than one clan tartan definitely sends the same message.