Simplify the following text for a Grade 3 native English-speaking person.
Original: `` You 're My Heart , You 're My Soul '' is considered their best selling single to date with worldwide sales exceeding 8 million copies and reaching the top of the charts in 35 countries .
Simplified: `` You 're My Heart , You 're My Soul '' is known as the best selling single of the duo . It sold over 8 million copies .