Bazze, metas automatically work together, they create a surface over a group of metaballs. If metas are grouped, they don't interact with other groups, the pos/neg attributes in the group don't have any effect, hence the need for the negative metaball. sorry should have been shift-click not alt for a negative meta.

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Toolset: Blender, GIMP, Indigo Render, LuxRender, TopMod, Knotplot, Ivy Gen, Plant Studio.