The two five-part series will spin out of the ongoing Buffy the Vampire Season Nine and Angel and Faith titles.
X-Men writer Victor Gischler will tackle the Spike miniseries, which follows on from Buffy Season Nine #10.
Issue #10 concludes the current arc of that title, and sees the slayer trying to make a clean break from the vampire.
"The focus is on Spike and his coming to terms with some things, sort of getting right in the head," Gischler told Entertainment Weekly.
"He loves Buffy, but can't be with her. What does that mean to a vampire with a soul?"
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Jeff Parker (Thunderbolts, Hulk) will write the Willow series, which sees the character tracking down Angel in an attempt to restore magic to the world.
Brian Ching will draw the Willow series with covers by David Mack and Megan Lara. Paul Lee will take on art duties for Spike with covers by Jenny Frison and Steve Morris, Comic Book Resources learned at the C2E2 retailer summit.
Spike and Willow will debut later in the year.