The following assumes lighting an additional candle for each day of Chanukkah:
Two may light in a single Menorah for the first three days of Chanukkah; each in his end of the Menorah. This does not work for the fourth day, since it would be impossible to tell if it is the fourth day (two people lighting) or eighth day (single person lighting).
Clearly, it is a requirement to be able to deduce the current day. (Kitzur Shulchan Oruch 139 (6))
Many Menoros have the Shamash dividing the eight candles into two groups of four:
Some have the Shamash offset to the side, but still have larger spacing in the middle.
How are we allowed to use such Menoros?