You will get the best results by running the first book discounted. But you'll still see sales of the other book if you run the second one at a reduced price. That's been my experience. So 
I never run an advertisement on my blurb for fear that Amazon will consider this a violation. Way back when, I read that there are concerns with Amazon TOS regarding this. But I'm unsure.
Okay here's my latest thoughts. A string of 99 cent promos on the second book and have the first book priced at 99 cents too. Then after a month, run a Freebooksy/Fussy Written Word Media free thing on
the first book. Then the next day, up the first book to $2.99 and make the second book $4.99.
I've been reading more and more of what you were saying about advertising the first book in a series. I know that's pretty conventional thinking, but I was hesitant to do that because it just worked well with my first book (making
it free after the run of 99 cent promos) that it was hard to shift gears on the Book 2. But I figure, if I want, later, I could run a free day/promo on Book 2 as well.
And I'll run some sponsored product ads for Book 2. And maybe some keyword ads for Book 2 too, but sponsored product ads work better for me (despite having no idea how they work!).