From my experience with these switches, you cant.
Any UNI port should be able to talk to a NNI port as can an ENI.
As you have stated, NNI can talk to NNI
If you need 2 uni ports to communicate, you need to use Private VLANS :-s.
Also check which sdm profile you are using with the show sdm prefer command as you will want to set it to either layer 2 or default (layer 3) depending on your needs. this will hit L3 performance if you are set to layer 2 :-
Me3400(config)#sdm prefer ?
default Default bias
layer-2 No routing