Create combined Lists from multiple Lists

It looks like what you're looking for is a connected component list. I answered a similar question about this here, but this question is different enough that I think it warrants it's own answer:

var combinedCoords = new List<List<int>>();
foreach(var c in coords)
{
    var merge = new List<List<int>>();
    foreach(var g in combinedCoords)
    {
        if (c.Any(g.Contains))
        {
            merge.Add(g);
        }
    }

    if (merge.Count == 0)
    {
        combinedCoords.Add(c);
    }

    merge.Add(c);
    for(int i = 1; i < merge.Count; i ++)
    {
        foreach(var v in merge[i].Except(merge[0]))
        {
            merge[0].Add(v);
        }

        combinedCoords.Remove(merge[i]);
    }
}

This produces two lists:

{ 0, 1, 2, 3, 4, 5 }
{ 7, 8, 9, 10 }

If you refactor coords and combinedCoords to be a List<HashSet<int>>, the algorithm is a little simpler, and it should perform better:

var combinedCoords = new List<HashSet<int>>();
foreach(var c in coords)
{
    var merge = new List<HashSet<int>>(combinedCoords.Where(c.Overlaps));
    if (merge.Count == 0)
    {
        combinedCoords.Add(c);
    }
    else
    {
        merge[0].UnionWith(c);
        for(int i = 1; i < merge.Count; i ++)
        {
            merge[0].UnionWith(merge[i]);
            combinedCoords.Remove(merge[i]);
        }
    }
}