Hmmm. Well, I guess we take these things on the chin eh? As presents go, I can think of ones much worse, but then I have to clarify that. as of course we're all cat fans here, so mainly we see it "double the pleasure".
I once left my stepdaughter's house with a small beige and white bundle which was one of her cat's kittens, and much as we hadnt looked to add to our kitty family, he was a huge blessing.
Not sure our existing cat saw it that way, especially when he decided to flatten the kitten whilst he was playing in a "lovely jubbly" champagne box, but on the whole it worked out okay.
I would say that in my view adopting two kittens is easier than adopting one, because generally speaking they will act as company for each other, thus leaving the older cats in (relative) peace, but providing enough amusement for them all to rub along. (But maybe I have a warped sense of reality....
)
I'd go for the "sneaking them into a spare room which has already been made kitten friendly" so they seem like a really intriguing mystery (in a good way) to your existing two. Make sure your two are not there to see the new two being secreted away in the chosen room. Let it be a surprise of the most magical variety.
Strange but interesting scents behind a door - in no way threatening, but in every way a mystery to be explored.
Plenty of playtime for each brace of cats, so no-one feels left out.
Gradually leaving the door open, so that sniffing at a distance can take place - scent swapping (blankets or toys) - and managed introductions over a period of time.
Do you have any names in mind as yet?