In theory you could give the same path to both plugins managers so that they install the plugins in the same directory:
In theory you could use one of the managers to install your plugins (
PlugInstall for vim-plug or
PluginInstall for Vundle), this plugin manager would then git clone the different git repository in
To update the plugins in theory everything should be ok: you would use either
PluginUpdate (Vundle) or
PlugUpdate (vim-plug) to list these repository and use
git pull in them, updating your plugins.
Where things might get fuzzy would be when Vim will source your vimrc: if you keep both
call plug#begin and
call vundle#begin, both plugins manager will try to load the plugins. It should work fine for the first one, the second one however could have some issues if the plugins don't have the right guards against being loaded several times.
In practice the real question is: Why would you want to do that? There is not pros to using two plugins managers: Is there is one plugin manager which is the only one to provide a feature you really need you should switch to this one completely. Keeping two in parallel will probably create more cons than pros.