According to the documentation for Configuring Cisco StackWise Virtual
The SVL link needs to be connected to the sup-module.
StackWise Virtual Link (SVL) connections are established only through 10G or 40G uplink ports on the
supervisor module. You cannot establish an SVL using a 10G port on one switch and a 40G port on the
Please refer to the following Cisco guide for details.
Although this is not a best practice, however, the following link contains the requirements.
Configuration guide for Cat 9400 and StackWise Virtual Link "configuring_cisco_stackwise_virtual.pdf".
And this is the design guide(CVD)
The following link is CVD "CVD-Campus-LAN-WLAN-Design-Guide-2018JAN.pdf".
CVD is a document that describes a verified configuration. It will be possible to find Best Practice. For deploying devices to the network
This also contains information Cat9x and StackWise Virtual Link.
As I note in the comments, I do not know if the answer to stack overflow is only to allow link paste.
However, I know that finding valid information on the official Cisco site is difficult.