I have an 8" F4 and I get the double spikes as well. I get them though on all 4 points. The fact you get them only on 2 could indicate your collimation is off ie. I used to get them on 2
before I learnt the art.
I never worried about the double spike, I actually use this as my measure of focus, if I dont get them then i am out.
Here is a sample
http://www.iceinspace.com.au/forum/a...se.php?a=87453