Donepezil - Can 10mg be cut in 1/2 if causes more side effects, I have IBS?

If they are not extended release (XL-XR-ER) and are not coated and are scored then yes otherwise no.
Also if they are sustained release (SR) and controlled release (CR) you can not split them in halves.

"Extended-release dosage consists of either sustained-release (SR) or controlled-release (CR) dosage. SR maintains drug release over a sustained period but not at a constant rate. CR maintains drug release over a sustained period at a nearly constant rate."

This means that if you split in half one of these pills, too much of the active ingredient will be released at once causing most likely potential side effects. I do not want to scare you at all, just giving you the proper information as for you to wean off your medication.

Do inform your Dr., of your discontinuation schedule.

The literature on Donepezil states:

"You should not stop using donepezil without your doctor's advice."

https://www.drugs.com/mtm/donepezil.html