The problem seams to be that you’ll have to join the both armatures together and then parant it to the body. then set all the vertecies to the corect bone and voila.
Yes, only one armature per object. (You can parent multiple objects to the same armature though.)
If you haven’t read the 30 + 30 min tutorial (“the gingerbread man tut”) in the Blender User Documentation where you give Gus a simple armature you should.
There is one small error in the tut: step 2 of rigging (adding the leg) says SPACE>>Add>>Armature when it should read SPACE>>Add>>Bones.
You add an armature for the first bone. Thereafter, to add more bones make sure you have the armature selected and tab into edit mode (you should never have left it for the Gus tut) and add bones.
Once you’ve added all the bones, tab back to Object Mode, select 1st your mesh then 2nd the armature and Ctrl-P to make parent. This will give you a little menu. select “Armature” and choose however you want to manage vertex groups.
Thereafter, to manipulate your character (mesh), always do it to the armature. Move, scale, deform, etc. The mesh will follow the armature as appropriate.