Complex sentence: It would mark the first time a solo artist had two songs in the top 5 and, the following week, Summer would be the first solo artist to have two songs in the Billboard top 3 at the same time.
Split into simpler sentences: It would mark the first time a solo artist have two songs in the top 5. In the week to follow, Summer would be the first solo artist to have two songs in the Billboard top 3 at the same time.