Shifting Borders — ruling 1

You reveal all cards you intend to splice at the same time. Each individual card can be spliced only once onto any one spell.

Shifting Borders — ruling 2

A card with a splice ability can’t be spliced onto itself because the spell is on the stack (and not in your hand) when you reveal the cards you want to splice onto it.

Shifting Borders — ruling 3

You choose all targets for the spell after revealing cards you want to splice, including any targets required by the text of any of those cards. You may choose a different target for each instance of the word “target” on the resulting spell.

Shifting Borders — ruling 4

If all of the spell’s targets are illegal when the spell tries to resolve, it won’t resolve and none of its effects will happen.

Shifting Borders — ruling 5

If one of the targeted lands isn’t a legal target as Shifting Borders resolves (say, because it has left the battlefield or become untargetable), the exchange doesn’t occur.

Shifting Borders : règles, fonctionnement et interactions (MTG)

Shifting Borders is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Exchange control of two target lands. Splice onto Arcane (As you cast an Arcane spell, you may reveal this card from your hand and pay its splice cost. If you do, add this card's effects to that spell.)

How it works : Shifting Borders

Original English text — Wizards of the Coast / Scryfall

#1June 7, 2013· Wizards of the Coast
You reveal all cards you intend to splice at the same time. Each individual card can be spliced only once onto any one spell.
No other card is named in this ruling.
#2June 7, 2013· Wizards of the Coast
A card with a splice ability can’t be spliced onto itself because the spell is on the stack (and not in your hand) when you reveal the cards you want to splice onto it.
No other card is named in this ruling.
#3June 7, 2013· Wizards of the Coast
You choose all targets for the spell after revealing cards you want to splice, including any targets required by the text of any of those cards. You may choose a different target for each instance of the word “target” on the resulting spell.
No other card is named in this ruling.
#4June 7, 2013· Wizards of the Coast
If all of the spell’s targets are illegal when the spell tries to resolve, it won’t resolve and none of its effects will happen.
No other card is named in this ruling.
#5June 1, 2005· Wizards of the Coast
If one of the targeted lands isn’t a legal target as Shifting Borders resolves (say, because it has left the battlefield or become untargetable), the exchange doesn’t occur.
No other card is named in this ruling.