Workflow field update overwriting value instead of adding value

Workflow field update overwriting value instead of adding value

Hi,

I have two workflows which are triggered when a ticket is created:
1) when the ticket description contains the word 'ziek', it should update the field 'Category' with the value 'Ziekmelding'
2) when the ticket description contains the word 'geld', it should update the field 'Category' with the value 'Geld'

The field 'Category' is a multi-select Picklist. However, it could be that in one ticket both words appear. In that case, the field 'Category' should have both values 'Ziekmelding' and 'Geld'. At the moment, the workflows are overwriting each other and replace the whole value instead of adding it to the multi-select picklist field.