I'm looking for some software or websites.

[Mabsark]

Does anyone know of any software or websites that allow you to input 2 bitcoin addresses and find a chain of transactions between the two if there are any?

[Flashman]

I was thinking searching "six degrees of separation" and bitcoin or blockchain would find something, but nothing obvious so far...

Here's an article which suggests things might be getting more complex in that regard..
http://www.coindesk.com/network-analysts-view-block-chain/

If it's worth money to you though, I know someone I can point this way equipped to do blockchain analysis.