hi. I don't do your type of kickstart but I'll explain my types of kickstarts and hopefully you can align that way of thinking to your type.
What I've been doing a lot of in the past couple of years, is kickstarting project templates into our workfront. Picture this:
* Each project template has many template tasks
* I list all the templates in one spreadsheet tab (the template tab)
* I list all the tasks in another spreadsheet tab (the template tasks tab) and reference the template line items in template tab
* I upload the one spreadsheet with two tabs
* templates are created and populated with template tasks
You should be doing something similar. What you have is a situation where each issue has many assignments.
* All your issues should get listed in one tab (the issues tab)
* All your assignments should get listed in another tab (the assignments tab)
I can't help you with whether you need a primary assignee to show up on the issue tab, or in fact any syntax at all. What I CAN suggest, and how I learned to do the templates, is that I downloaded the templates and template tasks kickstart from my instance. This provided me with MANY examples of how to fill out each cell. So if you already have issues in your instance with multiple assignments, you should just EXPORT a kickstart with issues and assignments, and figure out how it is being filled out. Please feel free to continue to use your post to ask questions about specific columns you see--my guess is usually to try and see what the bare minimum is, that I need to fill out. So I would personally try an assignment row with the assigned to ID, and the issue ID -- and add a column at a time from there.