I wanted to make a glass style double door that opens (for accessing areas enclosed by glass). The animation sort of works fine (for 1 animation action). I'm using Blender for the models and have 2 separate bones that are supposed to operate at the same time.

I run into this error:
door_open : 0.00 0.00
ERROR: calcblend failed in door_open
ERROR: Aborted Processing on 'glass_door_96x96.mdl'

It seems that this coding doesn't work:
$sequence idle "glass_door_south_left"

$animation a_door_open_left "door_open_left" subtract idle 0
$animation a_door_open_right "door_open_right" subtract idle 0
$sequence door_open {
a_door_open_left a_door_open_right
}

The model is a double-door that is supposed to pivot/translate open from a closed position.

Can't you animate both door bones in one SMD, instead of combining two separate ones?
(In Blender: move the timeline to 1, set a keframe on both bones, move time forward, rotate bones, set keyframes again, export.)

I have never used $animation before (just standalone $sequence lines), but from quickly browsing the documentation I noticed that you probably need to use "delta" on the door_open sequence.
(Check the $sequence and $animation pages on the wiki.)