Two actions with separate BLENDINs

I want my character to perform two actions independently: the lower group of bones is in charge of the walking part, and the upper bones does the aiming part moving the gun towards the enemy.
To do that, I created two groups of actions using different channels. First, I scripted the walking actions (forward/backward in “loop stop” mode) for the legs with a blendin value of 10 to get a smooth transition between them. (so far so good).
To get the aiming part, I set a “property type” action: the script changes the value of the action to rotate the character until the gun aims to the enemy.
The problem is that when I added the property action (for the gun) the “blendin” effect on the legs stopped to work. It is like the second action overwrites the blending action of the lower part.
One possible solution would be to use two different armatures, but the synchronization between them is really hard: I tried parenting the upper armature to a single bone of the lower one but this option is not available (just for a mesh and an armature).

Any hint ?

Regards,
SL