Another "can I do this! TC q

Hi

Have to put my 400 5.6 or 300 f4 on hold (partly finance & partly unsure which) so as an interim measure I would like to stack tcs on my 70-200 f4

I have a Canon 1.4 extender....I know I cannot use 2 together but after a bit of a read through here I think I can add a Tamron or Kenko.
I would like to keep autofocus.
Any help appreciated.
Thanks