Can one use a 2nd external monitor together with the new iMac 5k?
Yes. According to Apple's website, under the device's "Connections and Expansion" section:


  Headphone port
  SDXC card slot
  Four USB 3 ports (compatible with USB 2)
  Two Thunderbolt 2 ports
  Mini DisplayPort output
  Support for DVI, VGA, and dual-link DVI (adapters sold separately)
  10/100/1000BASE-T Gigabit Ethernet (RJ-45 connector)
  Kensington lock slot


iMac with 5K retina display specs