#\Theme::instance()->set_partial('subnavigation', 'admin/partials/settings_subnavigation');
\Theme::instance()->set_info('use_module', 'main', 'active');
\Theme::instance()->use_modules()->set_partial('subnavigation', 'admin/partials/settings_subnavigation')->use_modules(true);
\Theme::instance()->set_partial('subnavigation', \View::forge('othermodule::this/view/file'));Since this creates tight coupling between the two modules (because of the hardcoded name in your code), it is frowned upon. But it's your code, so you can use it if you want.
\Theme::instance()->set_partial('subnavigation', \View::forge('Main::admin/partials/settings_subnavigation'));
\Module::load('main');
\Theme::instance()->set_partial('subnavigation', \View::forge('main::admin/partials/settingssubnavigation'));
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