If (you might think,) just as (the eating of) offerings is time constrained (i.e., peace-offerings may be eaten only for two days and one night) so chullin is time constrained; it is, therefore, written (Ibid. 21) "with all the desire of your soul" (connoting whenever you wish).
If (you might think,) just as (the eating of) offerings is confined within certain bounds, so, chullin; it is, therefore, written (Ibid. 22) "the unclean one … may eat it."
This tells me only of the unclean one. Whence do I derive (the same for) the clean one? From "the unclean one and the clean one." "together may eat it" We are hereby apprised that both of them may eat it from one dish. I might think that terumah, too, may be eaten from the same dish; it is, therefore, written "may eat it (chullin)." This may be eaten from the same dish, but not terumah.