In his book about Kanban, David J. Anderson shows us how we can visualise activities on a Kanban board that can be done in parallel.
What to do when the sequence is not specified
2 Patterns for Parallel Activites
For both patterns it might be necessary to label the two cards to be able to link the two cards. For example: write the ID of the parent activity on both cards as a cross reference.
Pattern #1 – The Single Column
Only use one column on the Kanban boat that contains cards for both (parallel) activities.
Pattern #2 – The Vertical Split
Separate one column horizontall, one for each activity.
Source: Kanban: Successful Evolutionary Change for Your Technology Business by David J. Anderson, Mapping the Value-Stream (Chapter 6)