Does two different half HE equal 1 HE?

I had one piece of bread as my HE for lunch today (as thats all i fancied. Tonight i had a hifi bar do i need to count this as 6 syns or can it just be 3 and half a healthy extra?

You can split Hebs if you like so you could do as you suggest, or count the Hi Fi as the Heb and count 3 syns for the bread - it works out the same.
I split my hebs all the time like that, as long as you don't get confused, it's fine.