The passuk in Parshas Reeh states, “lo tisgodedu,” which Chazal learn to mean that Klal Yisroel should not make themselves into separate groups, performing different minhagim, in the same place. This halachah is the source that on Chol HaMoed, those who wear Tefillin do not daven in the same minyan as those who do not wear Tefillin on Chol HaMoed.
Rabbi Aharon Dovid Singer
Rosh Kollel, NMB Kollel