A biallelic knockout should be possible, but you will have to screen a lot of single clones that have to be grown to respective populations. I think my colleagues from a different group prepare minimum one 96 well plate, sometimes even 3-5 plates with single cells that they then growth up till they can check which clones are the best.

I never did it on my own so I don't know about the gRNAyou will have to design, I think companies who supply CRISPR/Cas9 system generally talk about minimum 2-3 gRNAs to have a good chance to get one gRNA working.

And in the end, it also depends on your target gene/protein. If it is essential for cells, maybe after a successful knockout cell will not growth out at all. And this might not be a problem with the cell line (as for my colleague the same cell line was working with other CRISPR/Cas9 gene knockdowns, but for some of the genes, no cells were growing out in something like 5 different cell lines).