CombineRegions[regions_List] := ReplaceRepeated[regions,
   {a___, {id1_, region1_}, b___, {id2_, region2_}, c___}
     /; RegionMeasure@RegionIntersection[region1, region2] > 0 :>
    {a, b, c, {Join[id1, id2], RegionUnion[region1, region2]}
     }
   ];