Unknown by Dell. As you can see in the image below taken directly from the monitor page, the Apple proprietary hardware, operating system (including Windows bootcamp), cabling, and drivers were not tested/validated by Dell.
Since we at Dell are not trained in the usage of the Apple hardware and operating system, you should ask the Apple users on the Apple Forum for troubleshooting and cabling opinions.
I am also having problems with the buggy DP and a Precision 4600m. I have a pair of U2414H, in fact I have 3, but I am only using 2, which have always been a little temperamental, but DP chaining worked ok with my 4600m, Windows 7 x64 and the ATI M5950. They are the older A01 rev, I bought them new and didn't know they had problems, I read else where Dell were RMA'ing them to upgrade the firmware which is supposed to fix a lot of the DP issues, they are now out of warranty and were when I found out... great :-/
Yesterday I decided to test KDE neon from a live USB stick but found that DP chaining wasn't working and plugged in an HDMI cable to test, which worked fine. When I went back to Windows and plugged my DP cable back in, both displays have decided to mirror, with the second monitor not showing up in device manager/screen resolution, nor the ATI catalyst control center. Both monitors work as expected when plugged in direct via DP. Display Port v1.2 is enabled on the first monitor in the chain, although it was enabled on both when working perfectly.
I have tried:
Different display port cables
Different monitors, I have a few U2415's.
I have uninstalled and reinstalled the Video drivers, trying both Dell and the more recent AMD versions.
Can anyone clarify, if I remove the ATI card, does the 4600m drop back to using the Intel HD graphics?
You will find DP chaining is unsupported by Apple, used to work in older versions of OS X, but like everything with Apple they think they know best ;-)