http://revach.net/article.php?id=2271

Section: Parshas HaShavua   Category: Quick Vort
Parshas Bamidbar: A Two Headed Levi?
Take a census of the entire assembly of the Bnai Yisrael.. Every male "L'Gulgalosom - according to their head count."  Why is the word L'Gulgalosom used in connection with the count of the Bnai Yisrael, but not for the counting of Shevet Levi, further in the parsha?

The Chasam Sofer answers that the Gemara in Menochas (37a) relates that Pelimo asked Rebbe on which head a 2 headed person should put his Tefilin. Rebbe considered the question to be obsolete and told him to either go to Galus or into Cherem (excommunication). Underlying Rebbe's harsh reaction was the view that such a person is a Treifa who can not live past 12 months, much less to the age of putting on Tefilin.

Therefore, concerning the counting of the Bnai Yisroel, who were counted from the ages of 20 to 60, this question could never apply. However, for the Leviim, who were counted from the age of 30 days, the question does apply.. Therefore the Torah omitted the word.  L'Gulgalosom to indicate that regardless of how many heads a Levi may have he should only be counted once.