Is it convenient to ask the journal's editor for two additional days to complete a review?

Getting a review in two days late for a journal article is pretty common. I wouldn't worry about anything less than being a week late. For conferences, deadlines can be tighter, but 2 days is nothing. Ideally you would let the editor know your review will be a couple of days late, but even if you didn't, it is not the end of the world.

I wouldn't even ask. Just "tell".

Send a short email saying you will send the review a week later (more time than needed, don't want to come back in 2 days again). Don't phrase the email in a manner that requires a response from the editor--tell, don't ask (cut the back and forth chatter).

If the editor wants/needs to pull the paper, he will do so. But it's very unlikely he will. But just let him do what he does.

In addition, I would be practical and do a simple review. I love doing extensive murder board reviews. But you need to concentrate on your own stuff. There is probably an 80-20 rule where you can give 80% of the key improvements with 20% of the text and parsing time. In particular, if you are recommending rejection, I would be economical with your time and not investment much in improvement recommendations.

When I ask for an extension, I usually ask for a substantial one, i.e. one week not just 2 days. This way, if I need more than 2 days for some unforeseen reason I don’t have to ask for another extension. Moreover if the editor is really tight she/he can negotiate down to 2 days and everybody is happy.

Dear editor, Due to unforeseen circumstances I can only guarantee submission of my report by[deadline+1 week]. I trust this firm new submission date is acceptable to the Journal.