Evolution and different number of chromosoms? (a non-biologist)
Speciation doesn't happen from one generation to another, it is a slow and continuous process, and for each changing that is kept by natural selection, there are millions of changes that resulted in less functional phenotypes and were negatively selected, and another million of changes that were lethal hence soon aborted (e.g. a mutation that causes an embryo to die even before a fruit or animal can get any development). So it might be possible that many chromosome number changes are taking plane in many organisms, but they  never will estabilsh in their populations. On the other hand, some numerical changes (and all the genetic processes that follow any change) may result in a good fit, and the individuals survive, establish and reproduce. But keep in mind that it is a slow and continuous process.

Theoretically, if among Chimpanzees, multiple offsprings outcome with 46 instead of 48 chromosomes:


In a theoretical scenario: They could be able to reproduce only among themselves (the 46-chromosomed-individuals) and, with time, accumulate some differences in relation to the 48-chromosomed-Chimpanzees, and them they would be named as another species.
In a theoretical scenario: In some conditions, they might be able even to reproduce with 48-chromosomed-Chimpanzees (e.g. if this numeric change resulted from fission of one chromosome, and when the two gametes meet, the two half chromosomes of one parental pair with the whole chromosome of the other one... That is not probable for animals, but happens a lot in plants)
If you are asking if they 46-chromosome chimps would crossbreed with humans... well, I believe they have accumulated too much differences in their genotypes and thus would no be compatible even if they have the same chromosome number. It happens that having the same number is not the most important factor allowing or preventing species to crossbreed. There are many barriers to hybridization, e.g. the lack of biochemical recognition between egg and sperm resulting in no embryo formation... in cases that embryo is formed, it may not develop, it may develop poorly, or it may develop but the offspring comes out compromised and live shortly. Another example that chromosome number does not prevent crosbreed is the mule, which is the offspring of a female horse (64 chromosomes) with a male donkey (62 chromosomes). That's because even having different chromosome numbers, they are still quite simmilar in a genetic perspective.


All that sayd, the odds that Chimps would loose two chromosomes, stablish in the population and reproduce with humans are really really really low... I would say that's impossible. But simmilar phenomena are possible and even quite common in plants (just so you know that it exists)

If you want to learn more about chromosome number changes, I recommend this link: https://www.ncbi.nlm.nih.gov/books/NBK21229/

(Modern Genetic Analysis. Griffiths AJF, Gelbart WM, Miller JH, et al. New York: W. H. Freeman; 1999.)