I'll try my best for ya Dave!
Both series in 2k and the meg were written as separate stories yet involved the same or similar plots. They were not written as one story where you had to get both titles at the same time, such as Wilderlands or Judgement Day.
Just told from a different perspective, that's all.
One was seen through the eyes of Demarco and the citizens of the city, the other from Dredd's point of view.
Doomsday for MC1 was printed in the Megazine, Doomsday for Dredd in 2k.
But both stories feature the build up to Doomsday, mention Dredd's capture and escape from that island where he was on trial and tell the battle to beat the droids and Narcos.
So you can read both books separate or together. It shouldn't make any diference.
Hope that helps.
scojo