Where is Twin Disciplines in the Priest talent tree?

Discipline tree, row 1, column 3. It is available as soon as you have your first talent point at level 10.

Is Twin Disciplines new in WoW Forever?

Yes. Twin Disciplines has no Classic 1.12 counterpart.

Rank	Effect	Source
1	Increases the damage and healing of your instant cast spells by 1%.	client
2	Increases the damage and healing of your instant cast spells by 2%.	client
3	Increases the damage and healing of your instant cast spells by 3%.	client
4	Increases the damage and healing of your instant cast spells by 4%.	client
5	Increases the damage and healing of your instant cast spells by 5%.	client

Tree: Discipline, row 1, column 3. Available from your first talent point. The earliest you can finish all 5 ranks is level 14 if you put every point into this tree.
Competes with: Power in Light, Wand Specialization in the same row.

Twin Disciplines is new in WoW Forever. Nothing in Classic 1.12 does this for Priests.

Shadow Priest leveling build (10–60) · 3/5
Discipline Priest healing build (31/20/0) · 5/5
Holy Priest raid healing build (20/31/0) · 5/5
Shadow Priest endgame PvE build (20/0/31) · 5/5
Discipline Priest PvP build (31/0/20) · 5/5