Is it correct to say Bible has 2 eyewitness accounts (Matthew, Mark and Luke as comprehensive one & John being the other) of Jesus Christ's lifetime?

The word "synoptic" applied to the Gospels means "taking the same or common view." So there is some truth to the idea that these three gospels present one view while John presents another. However, the idea that they present eye-witness accounts may be rightly challenged.

Mark and Luke do not claim to be direct eyewitness accounts but are second-hand testimonies at best. Luke acknowledges when he says in his prologue:

Inasmuch as many have undertaken to compile a narrative of the things which have been accomplished among us, just as they were delivered to us by those who from the beginning were eyewitnesses and ministers of the word, it seemed good to me also, having followed all things closely for some time past, to write an orderly account for you.

Thus, Luke affirms he has his material at second-hand and expresses a need to untangle what "many have undertaken to narrative" about his subject matter.

Mark was not a disciple and could not have been an eyewitness. Although Matthew was a disciple, the book is not called the Gospel of Matthew in the earliest manuscripts. It has no title and begins with the words "The book of the genealogy of Jesus Christ, the son of David, the son of Abraham." Calling it the Gospel of Matthew is a later tradition, although it was universally acknowledged by the Church Fathers who refer to it. In fact none of the Gospels was attributed to its traditional author in its earliest manuscript.

John is traditionally believed to be the Beloved Apostle. (John 13:21-30, etc.) The Gospel even states:

This is the disciple who is bearing witness to these things, and who has written these things; and we know that his testimony is true. (John 21:24)

So this is the Gospel with the best overt claim to being an eyewitness account. However it must be admitted that many scholars doubt that the Apostle John is the real author. Even if he is, the account has an overt theological agenda that makes critical readers question many of its stories.

To conclude, the only Gospel that claims to be an eyewitness account is the Gospel of John. So it is not correct to say we have two eyewitness accounts. At best we have one.

Because the Synoptic Gospels contain significant overlapping content but are not identical, we probably need to drill down one layer further.

In one sense, the claim is true: we have two Gospels written by eyewitnesses, those of Matthew & John.

In a more complex sense, though, there is a great deal of eyewitness content on Mark & Luke that isn't in Matthew or John. If, for sake of argument, Luke interviewed Mary (the mother of Jesus), to obtain his information about Elizabeth, the Nativity, the Magnificat, etc--then this is eyewitness testimony preserved by Luke--and only Luke.

Each Gospel contains details of the resurrection narrative that aren't found in any other Gospel (though Mark has only a smidge of unique content). Even if we set aside Mark for the moment, only Matthew tells of the soldiers guarding the tomb, only Luke tells the details of the appearance on the road to Emmaus, only John has the appearance at the sea of Tiberius, and so on. So we do in fact have more than two eyewitness accounts for some of Jesus' ministry. For the resurrection we've got at least four sources (and many would say five by adding Paul) providing independent but corroboratory evidence.

I should acknowledge that I do not believe in Markan Priority or Q, but I've certainly read thoughtful arguments by many people who do. One way that I've seen multiple attestation in the Gospels counted is:

1. Mark
2. Q
3. Material unique to Matthew (M)
4. Material unique to Luke (L)
5. John

And to treat each of these as independent sources. If a particular event is separately attested in all 5 of those subdivisions of source-criticism, it would be considered to be attested by 5 separate sources, not just 2.

My own theory on the Synoptic Problem (similar but not identical to the Griesbach Hypothesis) would rewrite the above as follows:

1. Matthew
2. Lukan content not found in Matthew
3. Markan content not found in Matthew or Luke (just a handful of verses)
4. John

So we can still get more than double attestation for some events.

Appendix

My own theory on the Synoptic Problem is presented here

For those who are frustrated by my extremely abbreviated claim that Matthew & John were written by eyewitnesses, a much more exhaustive discussion is found in the video series I'm making here.