How to salvage dry poster and acrylic paint?
Depends on the binder. If it's acrylic/latex, it can't really be reconstituted. For poster paint, there's a good chance the binder is gum arabic, in which case you can just add water, or grind it (on a slab with a glass muller, or failing that with a mortar and pestle) and then add water.

If you want to experiment, you can try grinding the dry latex paint. If you can get the particle size small enough, you could add the binder of your choice (acrylic again, gum arabic, heck even white glue could work) to make a lower-quality, but possibly still useful paint. But this is probably not worth the effort unless it was very expensive acrylic paint.