Assuming that they're on equal footing, NO CHANGE WOULD occur.
Say $1 = 100 Yen.
US money stock : $100
JApan money stock: 10,000 yen
Both decide to double money stock (devaluation)
US money stock: $200
Japan money stock: 20,000 yen
Currency rate: Still $1 = 100 yen.
This is...