Complex sentence: Despite their differences, the SDAP and Lassalle's ADAV shared a philosophy of socialism which was similar enough to mean that they were both routinely monitored and considered equally suspicious by the authorities.
Split into simpler sentences: Despite their differences, the SDAP and Lassalle's ADAV shared a largely identical interpretation of socialism. The similarity was great enough to mean that they were both routinely monitored and considered equally suspicious by the authorities.