Yōga Sūtra 4.20

एकसमये चोभयानवधारणम्।

ēkasamayē cōbhayānavadhāraṇam।

[एक समये च उभयोः अनवधारणम्।
ēka = at a single; samayē = time; ca = and / also; ubhayōḥ = both; anavadhāraṇam = it cannot wear;]

Also concurrently, it (Chitta) cannot wear two (topics).