Also @trurl, I understand perfectly how parity works, if you don't understand how 2 identical drives removed from an array would mean parity is still valid then maybe it's you who doesn't understand parity. The issue at stake was whether the drives being empty *were* in fact identical, that's what I wasn't sure of, I wondered if formatting would help, it wouldn't as @jonathanm explained, so I played it safe and am doing a rebuild.
*edit* I was being a bit facetious there as I'm sure trurl does understand how it works, but for anyone reading this who doesn't understand how removing 2 identical drives would mean parity was still correct and wants to know, I'll try to explain
Imagine you have 4 drives, you want to remove 2. As long as they are *absolutely* identical they cancel each other out in terms of parity, in the example below 3 and 4 are the identical drives
1 2 3 4 P
1 1 1 1 0
0 1 0 0 1
1 0 1 1 1
0 0 0 0 0
1 1 0 0 0
So if you remove 3 and 4 you are left with
1 2 P
1 1 0
0 1 1
1 0 1
0 0 0
1 1 0
As you can see parity is exactly the same. The reason for this is the value for parity is always whatever is needed to make the sum of the drives even. The sum of 2 identical drives is *always* even (0+0 = 0 | 1 + 1 = 0), so removing them can't have any effect on the parity value, if parity is 1, then 1 + 0 is still 1 and if it's 0, then 0 + 0 is still 0.
How you can be *absolutely* sure they are identical is another question, as if they aren't 100% identical you will have invalid parity, so it's very dangerous