Being a Muslim, i know that Islam holds all other religions within, since for example, Jesus in Christianity and David in Jewish are prophets in the Islamic religion. And we believe that Islam is the over all of all religions, so believing in Islam is believing in the other religions, the true soul of all religions
I am a Transcendologist who believes that truthfulness and rationality in religions are truths that can be substantiated by science or those that cannot be proven to be wrong. Spiritual interactions can only take place between a spiritual existence, if one exists, and our spirit. These interactions are then translated by our pre-conditioned minds and hence there are various religions.
I can think of a couple reasons that belief in multiple religions could be justified.
1 - Perhaps you believe God is a single entity, but subjective in that it materializes differently for everybody. Person A is a firm believer in Christianity, and after death he encounters figures from his belief system, such as Jesus Christ. Person B is a firm believer in principles such as Reincarnation and Karma, and after death he is born again. Both experiences are seen as equally valid, and one can therefore state that whoever holds this view believes in both religions, but he or she does not necessarily adhere to both. There is a quite a bit of debate and variation in polytheistic specifics, but this is a form of polytheism, one which posits that all Gods are the same God.
Proponents of a similar polytheistic belief state that "all paths lead to the same place."
2 - Perhaps you believe God is a single objective entity, but different religions interpret it differently. Have you seen or read "the Life of Pi?" In this book (and its adaptation) the protagonist becomes an adherent of 3 different religions (Christianity, Hinduism, and Islam). He attempts to understand God through "the lens of each religion," and before long he realizes that each has value and benefit. *Spoilers* At the end of the book, the main character Pi recounts his story to officials, who are skeptical of his tale about animals joining him on the lifeboat. They surmise that the animals represent people. Pi declares that neither version of the story explains why his boat sank at the beginning, and that neither can be proven. When he asks the officials which story they prefer to believe, they choose the one with the animals. Pi replies, "and so it goes with God." There is a meaning to this beyond the obvious allusion to Noah's Ark and the implication that the fantastical is sometimes ironically truer than the easily believable, which is that people make their own choices about what to believe, and that while some might even be wrong, it doesn't make the least bit of difference. Someone who comes this conclusion is likely to grant each religion some amount of validity, and they may even practice several at once. This is yet another form of polytheism.
There is at least one more example, but there is simply not enough room for it. It basically posits that all or many Gods are real, but separate (ie. Not manifestations or different perceptions of the same God, as in examples 1 & 2).
Whether or not you agree with these beliefs, they are perfectly valid. So I would have to say that, yes, it is possible to believe in more than one religion (even the monotheistic ones). A proponent of this kind of belief wouldn't be monotheistic themselves, but they could certainly include the most relevant aspects of monotheistic religions under their polytheistic belief system.
All religions cancel each other out which is why there are multiple religions. Being said that, religions give people the freedom to choose which religion they alone want to follow. There is no need to be monotheistic. You either believe in a certain religion or you dont' simple. No ands buts or or's.
Adhering to multiple belief systems would not be possible, as it would render one into a state of conflict. Although many religions contain similar truths, they conflict with one another in beliefs and practices.
For instance, if one were to adhere to the religion of Islam, and then adhere to Christianity at the same time, both religious beliefs which conflict with one another because of the absolute truths they both propose. In Islam, a believer is to believe in a totally transcendent deity, who, because of his total transcendence and absolute unity, could never exist as a complex unity. On the contrary, Christianity teaches that God is absolute one, yet the Divinity exists as three "persons": the Father, the Spirit, and the Holy Spirit. The God of the Christians is understood as being one in His nature (divine), which exists in an absolute tri-unity. Such a beliefs would be absolutely contrary to Islam. Therefore, one who claims to believe and adhere to both religions simultaneously, would be contradicting in his or her beliefs.
What it all comes back to is absolute truth. One who claims that "all paths lead to God" is claiming that all truths are equal and merely relative (i.E., there are no absolute truths). However, to make such a claim is the assertion of an absolute truth that there are no absolute truths.
Truth is not just a subjective thought or idea. Truth is a Person. Truth is God.
All religions have the same basic goal: union with the Divine.
However, God did this in the most intimate way, by humbling Himself -the Creator of all- by taking on flesh and becoming a man, and yet remaining God. The God-man Christ Jesus, the Son of God (in a metaphorical sense, alluding to His divinity). Through Christ's incarnation, He has created a bridge, a unity, a communion between God and man.
"I am the Way, the Truth, and the Life, no man comes to the Father but through me." (John 14:6).