Hi,
Can we enable .Net 3.5 Windows 2019 server along with . Net 4.7
Yes. These two versions are separated.
Note!!!
Not all versions are independent. For example, you cannot run versions 2.0, 3.0, and 3.5 side by side since these are all built with the same version of the CLR (CLR 2.0).
Only from .NET Framework 4 Microsoft ended this layering approach and you can use multiple versions side-by-side. So, you can have one version out of 2.0, 3.0, or 3.5 and multiple newer versions
Important! I highly recommend to check the following document regarding the .NET Framework requirements for various versions of SQL Server