Check out How to Launching Multiple Projects in JetBrains Rider by JR. Here is an excerpt:
Create a project for every IIS project you have in your project.
Configure one of them with a start page.
Add them all to a ‘Compound’ project.
Hit run and watch them go
Your multiple projects in a ‘Compound’ project
One of the project configured with a start page
The full article is available here.