It sounds like your concern is that each card (or pair of cards) will have a similar, linear effect on your credit score.
Some anecdata: Over the last 2 years, I have applied for 6 cards, approximately 4-6 months a part from one another. With the first pair of cards, my score also slipped 15-20 points, like yours, and crossed the threshold. The next card (a single one, not a pair like the first time) dropped it 5-10 points, and it crept up again. For the last three cards, the change was more negligible (5 points, I think). I started in the 810 range, and have never dropped below 785, even though I opened cards seriatim. I stayed in the ideal zone the whole time.
Note, my average age of credit stays fairly high because my oldest card has been active since about 2006.
The math on how credit scores work is a little opaque, but I think the answer is that you do not need to be concerned that your next card will make you drop to say, 770, and then next to 750 (out of the golden zone).