Is your feature request related to a problem? Please describe.
Let's say I have 2 project that use sql_sources: one project uses templater = python, second project uses templater = jinja. Running sqlfluff on all files produces the following warning:

WARNING    Attempt to set templater to jinja failed. Using python templater. Templater cannot be set in a .sqlfluff file in a subdirectory of the current working directory. It can be set in a .sqlfluff in the current working directory. See Nesting section of the docs for more details. 

Basically one invocation of sqlfluff can only use a single templater.

Describe the solution you'd like
It would be nice to run sqlfluff for different templaters in separate processes to fix this.

Describe alternatives you've considered
Support the feature in sqlfluff itself, but this might be more complicated.