They probably felt Batman was popular enough to warrant more than one series. It's like Spider-man. You've got the Amazing Spider-man, Web of Spider-man, Spectacular Spider-man, just Spider-man, then Sensational Spider-man and Superior Spider-man. Some characters are just so popular that even if they did more than one series, people would buy it. There have been times where a series would feature a particular artist or writer and sometimes a series has a multi-issue run where as the other series will feature supporting stories or unrelated stories, etc. but when it boils down to it, it's the money.

Ok cool. Just wondering if I need to keep jumping between thebtwo series ill stick with batman for now then look at getting dc

Yeah unless you run into one that says "Continued in Detective Comics #xxx" and you really want to know what happens, you should be okay.

Excellent. Cheers peeps