When dicing for the attacker (page 42/43) it states that the retinue with "the highest combined number of mounted yeomen and bidower units wins ties". I am assuming that if you have one bidower unit (or Mtd Yeoman) and the opponent has none then you win the tie. I.e. it is not a requirement to have both unit types in your retinue?
That seemms common sense to me but wargamers are seldom known for that trait!