Any ideas on this one?

Perhaps make two links in your menu, one called "DKP Standings" (or something) to http://followers.dkpsystem.com/dkpmember.php?dkpsystemid=2

And the second "30-day Attendance" that links to http://followers.dkpsystem.com/dkpmember.php?dkpsystemid=2&sort=member&fromdaysago=30&slid[]=1&slid[]=5&slid[]=20&slid[]=30&slid[]=50

Or you could just explain to your members that the first URL is the only way to check their DKP and if they want to see their raid% to click that tab?

Or you can do what my guild does and split DKP and Attendance into two different DKPsystems... but that's way too much work if you only need a simple solution.

At the moment the tabs are completely separate, though there are plans to try to allow users to join them to a degree.

But for now you'll have to switch between the different tabs to see both sets of info.