E-Port Plus II with Latitude E5250

What I though should be straightforward is proving rather difficult.

Latitude docked on E-Port, lid closed, powered on.

Two monitors connected via the E-Port's display ports. Get a picture on both monitors.

One monitor display at full resolution, the other won't go beyond 720 in the Intel software. However if I leave one of them in the E-Port and plug the other into the Latitude's DP socket, then I get both monitors at full resolution.

Any ideas?