Just got the Ancestry.com autosomal matches for G.B. and F.P. who are half siblings. They were tested under two separate accouts.
With a rather tedious process I copied, cleaned up, sorted then compared their 4th-6th cousin matches.
GB has 38 such matches. FP has 63.
Of this group they only have three matches in common. Initially I was surprised there are so few. But after thinking about it, the chance of any 4th - 6th cousins matching is low let alone a 4th - 6th cousin matching siblings.
I also quickly checked to see if any of them shared ancestry with any of the others but again the chance that two different 4th - 6th cousin matches also share common ancestry, especially common ancestry that is documented, is very very low.
I have messaged each but my response rate for such contacts is really poor so I am not expecting any breakthroughs.
I have not tried this with their more distant matches. Their trees are so shallow I can't be bothered with that right now.