The words "any electricity attack" suggest to me that they do indeed stack. I think that a shambler in a room full of shocker lizards will result in a bunch of dead SLs soon after the shambler gets hungry. The SLs are carnivorous, so they're not going to see the shambler as prey. The shambler is also carnivorous and is going to see the SLs as prey. When dinner time comes, the SLs, which are thick, will not realise that their electrical defence against being eaten is only bolstering the shambler. Of course, even thick SLs must have a basic survival instinct, so they might try to get out of harm's way. Then again, both naturally inhabit marshes (though the SLs prefer warmer ones), so the SLs might recognise shamblers as a menace to be avoided.