Complex sentence: Higgins married fellow artist Alison Knowles in 1960 and both took part in the Wiesbaden, Germany "Fluxus" festival in 1962 that marked the founding of Fluxus activity.
Split into simpler sentences: Higgins married fellow artist Alison Knowles in 1960. Higgins and Alison both took part in the Wiesbaden, Germany "Fluxus" festival in 1962 that marked the founding of Fluxus activity.