PSA and not enough ads rules

What happens if

[jetteroheller]

I have normal 2 ads on my pages.

I am just replacing the PSA by own ads.
There are some questions.

When there is a PSA in ad block 1, ad block 2 remains empty.

Is this the same when I replace PSA with own address?

When there is only one ad available from AdSense, ad block 2 remains empty.

Is this the same when I replace PSA with own address?

[jetteroheller]

When You place 2 times the same AdSense code
with PSA alternative URL, both blocks will show
the same add from the alternative URL.

So maybe I place in block 1 an add for my newsletter,
and block 2 remains with PSA

But this solution is not perfect

When AdSense fills only block 1 with an add,
block 2 should show the newsletter abo add

Current no solution to solve this problem

[jetteroheller]

To make it even more complicate:

Block 1: alternate URL
Block 2: PSA

No PSA shown

Block 1: PSA
Block 2: alternate URL

PSA shown