Here is the product..

I have previously applied a local urea fertilizer, rated at 46%, should be 46-0-0 type. Was applying 2kg per 1200sqft. The growth was enormous and chocked my hand reel mower! Is 2kg is too high? I saw there is a calculation too but unsure..

2kg sounds like a lot. I'd drop back to 1kg and see how it looks. Drop back more if you need to, but still apply monthly.

Or you could mow every 2 days to keep from choking the mower.

Yes @dchall_san_antonio, I will drop to 1 kg for monthly feed, I could save more too, Mowing frequency is still once a week for now. Time is a limit for me now! :(

Can I mix the above product and the 1kg of urea 46%? Any harm to my lawn?

You bought a bag of starter fertilizer, good stuff for starting new seed but isn't something to use on established grass. Cut back on the Urea as dchall said.

Will do. Thanks @outpaientzero

It is very interesting for me too

You can mix organic fertilizer with chemical fertilizers if you want to. You can apply both at the recommended rate at the same time, and nothing should happen. The organic fertilizer has a completely different interaction with the plant from the chemical fertilizer.

Since, I have purchased it, I'd rather use and finished it. Will apply monthly then. Thanks @dchall_san_antonio.