Out of pure curiosity, what is his argument in support of the second assertion?
One argument Volf presents is that there are a few people who are 'living' both religions. The famous case of the former Episcopal priest, a Ms. Redding
, was brought forward. But he also mentioned Muslims living in Asia who, though accepting Christ as the Son of God, remain outwardly Muslim (going to masjid, prayer five times a day, Ramadan, etc.). One such Muslim, Ibrahim, is quoted as saying that the Qur'anic verses supposedly denying the Trinity, divine Sonship, the atoning death, and the Bible's textual integrity, were "open to alternate interpretations" and thus he could follow Jesus and be a Muslim.
Theologically, Volf argues that a Christian adopts three essential beliefs/practices: (1) baptism in the name of the Trinity; (2) confession that Christ is the Lord, God in human form; and (3) reception of the Holy Spirit. A Muslim, he says, has four: (1) observing Ramadan; (2) five-times daily prayers; (3) reciting of Al-Fatihah; (4) belief that Muhammad was a prophet (not necessarily the last prophet, though).
Since the 3 Christian beliefs/practices do not directly contradict the 4 Muslim ones listed (e.g., one can confess that Jesus is Lord, while accepting Muhammad's prophethood, which would not mean that one would necessarily accept other beliefs that have become associated with Islam or with Christianity), Volf implies that it is possible to be both Christian and Muslim.