Simplify the following text for a Grade 8 native English-speaking person.
Original: Wembley had opened in London only a few months earlier , the near-simultaneous development being a source of rivalry between the northern and southern divisions of builders Sir Robert McAlpine , who built both stadia .
Simplified: Wembley had opened in London only a few months earlier . This made a source of rivalry between the northern and southern divisions of builders Sir Robert McAlpine , who built both stadia .