Using the example from the documentation:

--8<---------------cut here---------------start------------->8---
   (service slim-service-type (slim-configuration
                               (display ":0")
                               (vt "vt7")))
   (service slim-service-type (slim-configuration
                               (display ":1")
                               (vt "vt8")))
--8<---------------cut here---------------end--------------->8---

The first service works.
The second only shows a blackscreen however and uses 100% of a CPU.

-- 
Pierre Neidhardt
https://ambrevar.xyz/