I would use a boolean modifier set to union, apply it and use the sculpt mode to smooth the intersection. This will ruin the topology so if you want to animate it you will need to retopologize.
Alternatively you could merge the vertices as was suggested, or just keep them as two separate meshes and use clothing or necklace to hide the neck seam.
Help joining two objects together seamlessly (lemmy.world)
Hello I was wondering if someone could give me some advice on how to do a thing in blender...