I thought I had read it somewhere but cannot find it now, but do like-effects stack? For example, if I have two Shadow-Web Weapons attuned to me (spending the attunement cost of course), would I receive a total of +20 to my Assassinate feat?
Yes! Snake, Crowley, and a few Brotherhood have done it.
If you’re willing to pay for the weapons, upkeep the attunements, and devote both hands to that ability instead of whatever else you could pick up, you could even get a Shadow-Web shield for the same benefit.