David Warner

2001: Grim & Evil (animated series) uncredited voice work in "Fiend is Friend without the 'R'"
1995: In The Mouth of Madness: Dr. Wrenn, psychiatrist trying to help Sam Neil's insane character
1994: Necronomicon: Dr. Madden (segment 3)
1993: Unnamable II: The Statement of Randolph Carter: Chancellor Thayer
1991: Cast A Deadly Spell: Amos Hackshaw, summons Yog Sothoth

Honorable Mentions
1981: Time Bandits: Evil Genius (Minion Like)
"No one created me! I am Evil! Evil existed long before Good. I cannot be unmade. I am all-powerful!"
1979: Nightwing: Phillip Payne, vampire bats controlled by supernatural forces attack a small town, sounds kind of like The Hound by H.P. Lovecraft

Jeffrey Combs

2002: Beyond Re-Animator: Herbert West
1994: Lurking Fear: Dr. Haggis
1994: Necronomicon: H.P. Lovecraft (part 1)
1990: Bride of Re-Animator: Herbert West
1986: From Beyond: Crawford Tillinghast
1985: Re-Animator: Herbert West

Honorable Mentions
1988: Cellar Dweller: Colin Childress: If we think about giving Warner "Nightwing," then Combs get this one: A 1930's horror comic artist's creations come alive and kill him.

If we discount Warner's voice-only work in Grim & Evil, and Combs' work in the Reanimator sequels since it is the same character, we have a 4-4 tie.

Cthulhoid Factoids

Both were in 1983's "The Man With Two Brains" starring Steve Martin. Combs played Dr. Jones. Warner played Dr. Necessiter.

Supposedly "Pulse Pounders," filmed in 1988, will be released in 2002 which stars both Combs and Warner. Originally it was broken up into three shorter pieces. The one story that involved the "Trancers" character Jack Deth was expanded into it's own separate film. Thus leaving the Lovecraft piece(s) stuck in limbo. Out of space, out of time.