Full Idea
The Principle of Conjunction says that Γ |= φ∧ψ iff Γ |= φ and Γ |= ψ. This implies φ,ψ |= φ∧ψ, which is ∧-introduction. It is also implies ∧-elimination.
Gist of Idea
'Conjunction' says that Γ|=φ∧ψ iff Γ|=φ and Γ|=ψ
Source
David Bostock (Intermediate Logic [1997], 2.5.F)
Book Reference
Bostock,David: 'Intermediate Logic' [OUP 1997], p.33
A Reaction
[Second half is Ex. 2.5.3] That is, if they are entailed separately, they are entailed as a unit. It is a moot point whether these principles are theorems of propositional logic, or derivation rules.