I think it's a bit late in the day but I saw your topic only now..So here we go :-
I took the same approach as you and started with Maths and then added English a couple of months later. My daughter was in Year 3 and we started during April-May.
I don't think it's difficult to manage both unless you do other extra daily work with him apart from Kumon(we used to do additional Maths,bits of English/Science, some homework). For us, it did get a bit too much to handle on certain days for example ..say when she was starting on a new/difficult topic in maths or when she was tired. In these instances the work did lag behind a bit,we then did extra over the weekend.
I used to take a break(you can do that if you are doing both the subjects at the same time) and would suspend English for a month or two and resume it after that. One thing which can get boring with English, is the repetitions, unlike Maths my daughter remembers her English answers and furthermore I felt there was no point in her reading the same passage and answering the same questions 2-3 times,when she got them right the first time.
So I requested the Kumon tutor to give her each paper just once only.. After a certain set was over, say B1 we then took a month's break and then moved to B11. I guess you will have to find a pattern that suits you.
Now we have reached a point where I have suspended English totally (she is in level E in English now) and am concentrating on getting her through the Eleven Plus
Based on my personal experience and of a few parents that I know, Kumon Maths is a lot more helpful than Kumon English. Kumon Maths follows a set pattern which helps in developing basic mathematical skills tremendously. Kumon English just covers certain areas(like comprehension, where again recollection doesn't come into picture as you can read the passage again and find your answer) whereas your grammar skills(like tenses, adverbs, adjectives etc) are just skimmed over.That's my conclusion based on how far we have reached in English, I am not sure if it changes later on. But I would defintely put her back on English Kumon when she is in Year 6 , once all the eleven plus is behind us.