Question : can a Christian or Muslim be a follower of Buddha and continue practicing its faith ?
It would be difficult. The problems would mostly come from the Christian and Muslim ends, though. Islam particularly, would probably take a pretty dim view of that.
Buddhism hasn't traditionally denied the existence of gods. Gods are portrayed as participants in some of the canonical Buddhist writings. They are said to have gathered to honor the Buddha's birth and death, for example. (Historians of Indian religion take great interest in which gods are mentioned in the early Buddhist writings, what their attributes are and so on.)
But Buddhism doesn't have a mono-theistic-style creator god. The universe is imagined as effectively being of endless duration, without any beginning or end. There isn't any lawcourt-style postmortem judgement. Rewards and punishments are attributed to karma, to causality essentially. Morality is an integral part of the law of nature for traditional Buddhism.
In other words, while gods may exist, they are basically something like space-aliens, creatures as we are, just more powerful and exalted. Buddhists don't assume that human beings are the pinnacle of creaturely reality. But the Buddhists' gods are mortal, if extremely long-lived, and they are still subject to the law of karma. They can eventually die and even effectively fall out of heaven into an earthly or even a hellish rebirth.
(Given that each of us is the result of an endless series of rebirths already, everyone probably has gods somewhere in that history.)
In Buddhism, gods don't really have very much religious significance. They are still in need of enlightenment themselves. That's why they are portrayed as recognizing the importance of the Buddha's birth.
So Buddhism has never had any big problem in incorporating other religions' deities into its scheme. (More gods with new names, so what?) Probably none of the gods found in Buddhist writings originated historically as Buddhist deities. They are just the deities that were already popular among the general population at the places and times when the writings were written.
But religions that imagine their god as being the one true god, ultimate being, creator of the entire universe, postmortem judge, the final goal of man's religious quest and of all of history, probably will have quite a bit of difficulty accepting the Buddhists' devaluation of their deity. They certainly will have problems accepting the idea that true salvation has nothing to do with their god and that their god stands in need of Buddhist enlightenment just like we do.
Having said that, we do see things like "Christian Zen" here in California. (Perhaps more so in the 60's and 70's than today.) This tendency adopts the outward aesthetic style of Zen (perhaps hybridized with Western contemplative traditions), but interprets Buddhism as a psychology, a this-worldly meditation practice conducive to peace and equanimity, while still retaining the idea that true religious salvation comes through Christ alone. Buddhist enlightenment is no longer imagined to be the religious summum bonum. In this case it's Buddhism that appears to be the faith that's been devalued. But Buddhism never seemed to have any problem with the Christian Zen people.
We also see some Jews who adopt some Buddhism without giving up their self-identity as Jews. Many of these are non-religious Jews for whom Judaism is no longer their religion, just an ethnic identity. Others seem to treat matters like Christian Zen does, meditating while still believing in the Jewish god.
But I've never heard of any Islamic-Buddhist hybrids. They may exist, but I'd guess that it would be awfully close to idolatry in Muslim eyes. The community's reaction against it might conceivably be violent. I'd speculate that Muslims who might be attracted to Buddhism are more likely to opt for Sufism instead.
