SW say it's fine to- this is lifted directly from their FAQs
Q: Can I mix and match my Healthy Extras by halving the portions?
A: Yes, if you like - you’ll still lose weight beautifully! The downside is your speed and fibre symbols would have to be ignored unless your two halves had identical symbols. However, mixing and matching couldn't be done when there are multiple items in one choice, e.g. 28g Kellogg's All-Bran 1 Ryvita Multigrain crispbread. With all single item choices, feel free!
To keep the food diary as easy as possible to update, our Healthy Extra drop down lists only allow you to enter a whole Healthy Extra. If you do enjoy two half portions and want to record that, please make an entry in your food diary journal.