Some people do split their hex allowances, but I always advise against it. SW give the option listed under the healthy extra choices as they are what gives you the recommended daily fibre and dairy intake.
For the hex b choices, there is a specific list of 'grab & go' options that will ensure you get the full hex b fibre recommendation.
For example, 1 Weetabix and 1 alpen/hifi bar
I haven't got my book with me so I can get the full list (it doesn't show them on the website) but I'll check it when I get home