In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.

Passage: During Tool's post-Ænima hiatus to deal with their legal issues, Keenan began working with Billy Howerdel, Tool's guitar tech through the Ænima tour, on a different project. The supergroup they formed, A Perfect Circle, began performing in 1999 and released its first album Mer de Noms in 2000. They released a successful follow-up in 2003 titled Thirteenth Step, a reference to twelve-step programs (many of the songs were written from the perspective of recovery). Both albums were eventually certified platinum. Their subsequent 2004 album, eMOTIVe, was primarily composed of covers, except for the singles "Counting Bodies Like Sheep to the Rhythm of the War Drums"—a song inspired by "Pet" that was originally released on Thirteenth Step—and "Passive". Keenan later characterized the record as a political album with which he "tested the waters" and was subsequently "crucified" for it because of the content. It was certified gold the month after its release. That same year they released the DVD and CD set entitled aMOTION, which was certified platinum within a month of its release.
Howerdel reported in a May 2006 interview with MTV that the supergroup's work was concluded for the time being. After more than two years since the band's last release, Keenan was asked about the status of A Perfect Circle during an interview with Revolver. He stated:
The real problem with running Tool and A Perfect Circle at the same time was they both operate the same way. They're both live touring bands with a label, still working under the old contract mentality. So I thought it was time to let A Perfect Circle go for now and let Billy explore himself. It's tough for a guy who went from being a guitar tech [for Tool] to being in a band with a pretentious, famous singer and having to live in that shadow. It was important for Billy to go and do his own thing and really explore his own sound and let people hear what he has to say and how he would do it on his own, and then we'll get back and do some A Perfect Circle stuff.
What are the names of the two people who formed the supergroup?