I CANNOT believe I am rec'g this again, but in the Bittersweets Series by Herself, there is a story late in the series where Spike and William are in the same place and the interaction and reaction between the two is interesting. Be advised that it's a long and ansty series so if you want funny-fluffy this set of stories is not for you. The story with the William/Spike is:
Mrs Grieves & The Abandoned Husband by HerselfA story set in The Bittersweets-verse
http://www.echonyc.com/~stax/Buffy/herself/Husband.htmlSummary: "Don't come all snooty with me. I know you, I do." Spike gripped his wrist tighter. "You're tragic, you are. No—no, not even tragic. Not good enough to be tragic." I highly recommend reading the entire series, but this is the story that fits your bill.
Then of course there is:
Double Spiked by kantayrahttp://dark-solace.org/elysian/viewstory.php?sid=523&warning=NC-17Summary: When the nerdy trio delves into time travel, Buffy is faced with two Spikes at once. Will she be able to resist? In the all human 'verse, there is of course no time travel or mystical mistakes, so the double thing winds up being twins or cousins or what have you. In that vein:
The Hunters by greyangelhttp://spikeluver.com/SpuffyRealm/viewstory.php?sid=29306&warning=NC-17Summary: William and Spike are twin brothers. Spike lives in England with his older brother, Angel and their father, Giles. Estranged from his family, William has settled in LA, where he's married to Buffy. But, unbeknown to Buffy, William has another life back in England, though he hasn't seen his British family in six years. Time passes, and life is happy if a little hard for the couple...then one day, William's twin brother, Spike turns up asking William to return home with him. With the arrival of Spike Hunter, Buffy's idyllic life is about to change forever. This one was never my thing although I think it is quite popular and it fits your criteria. It is followed by the primary stories which make up the Twinlets universe. It is all human:
Singlets by Xela Summary: Pre-qule to Twinlets: Spike and Will, two sucessful publishing VPs, are looking for a roommate. Buffy Summers, a grad student at UCLA, just happens to be looking for an apartment.