You can run one external display at 3840x2160@60Hz, or two external displays at 2560x1440@60Hz. There are two thunderbolt connectors, but only one thunderbolt controller, which only has the bandwidth of a single DP1.2 port. If you connect two UHD (3840x2160) displays, I'd expect both of them to run at 30Hz.
Contrary to my expectation, I have two UHD monitors running at 3840x2160@60 Hz connected to a Mid 2014 15" Retina MacBook Pro (
MacBookPro11,3). Both external displays are
Samsung U28D590 with DP1.2 enabled. There are two separate ways of displaying the resolution and refresh rate (the monitor's own report, and
System Preferences), and for both monitors, 3840x2160@60 Hz is reported by both methods. To see all of the details in
System Preferences, hold Alt (Option) while changing
Default for display to
Scaled, then select
Show low resolution modes. With my displays, it doesn't allow me to change
Refresh Rate, but it displays as 60 Hz on both displays.
On a separate note, I recently discovered that the Apple Store claims that a 27" Retina iMac can drive two external UHD displays at 60 Hz, or a single 5120x2880 display with dual inputs. When I first saw that I wondered whether the newest Retina iMac has a second Thunderbolt controller, but today's test on my Mid 2014 MacBook Pro suggests another possibility, a software improvement that somehow makes better use of the available bandwidth. I'm not convinced today's experiment would have had the same outcome a year ago, but I'm not quite tempted to install an older release and repeat the test (this MacBook Pro is running
MacOS X 10.11.4 (15E65)). If anything, I'm more tempted to buy a Dell UP2715K to see if anything I already have can drive that.
I have neither a DELL P2415Q nor a 13" MacBook Pro to test, but based on today's experiment I'd say there's a good chance your Mac can drive two P2415Q displays at 3840x2160@60 Hz. Failing that, the second display can probably do 30 Hz with the HDMI port.