Only a guess from a non type rated person...
If you had two packs operating from a single bleed, then had an engine failure of the engine with the bleed failure. You'd be trying a run an engine feeding both packs at MCT thrust and maybe that would mean that the core of the engine may be working a little too hard and single engine performance wouldn't be met... Especially if you drifted down to a level where wing/engine anti ice was needed too.