MIDI envelopes and channels (X2 Producer)

Hi, i could be wrong however, as far as i know Volume Envelopes deal directly with the output of the synth. So if you have one synth with one volume output, there can only be one envelope to control that volume. 
 
For example, if you use Kontakt, with 16 outputs, you can have 16 midi channels controlling 16 instruments with 16 envelopes (ie CC 11 - expression for each and every instrument/midi channel and or CC xx - other controller... etc x16). 
 
I dont use Battery 3 however, again assumption, for drum synth the entire keyboard triggers different sounds but they all go out via a single midi channel, thus you may be limited to a singular envelope control. 

As you mentioned, you have split the keys into separate tracks. The only way i can think of for automation is to use "Clip Automation > Velocity". It is selected on the midi track on the main track view.
 
This WILL NOT give you control to volume automation per se; however, it works on controlling the velocity of midi notes contained in your track. 

hope it helps....

Hopefully I am reading your post correctly. I am not a MIDI expert and don't have battery so bear with me. If you are using a midi volume envelope it is going to affect the entire drumkit on most plugs because the snare and kick are still likely to be on same midi channel. Assuming Battery is multitambral, you would need to make a kit with components going out as separate midi channels, which seems like more work than it's worth.

Or

Have the audio of kick and snare routed to separate tracks and create an audio volume envelope instead. Unless there is some MIDI specific trick you need to do with volume, this seems to be the easier to implement option.