How to work with 2 Applications folders?
Hints for solving your XY problem


Using an external hard-drive to store your applications is not recommended and will slow down their execution. Unless you are talking about a thunderbolt 4TB Solid State Drive, performances of your applications running on the external hard-drive will be very poor (especially if you compare with your internal SSD).
Try as much as you can to move your movies, pictures, documents out of your SSD and keep your apps there to make the most of your SSD. You don't need a big data read/write rate for: watching a movie, browsing photos or pdf.
If you are a professional and need to process photos/video, then a thunderbolt SSD will be life-saving. 


Solution


If you don't mind about the three points above and stick to your idea, to come back to CoderX very bad answer, the proper solution to handle 2 applications folder is:


to keep your original /Applications folder.
Create a folder on your external hard-drive. 
Create a symbolic link for each application stored on your external hard-drive into your original /Application folder. 



Example

&lt;InternalSSD path&gt;/Applications/ contains: 

drwxr-xr-x   3 root       wheel  96 2018-06-16 09:18 app1.app/ 
drwxr-xr-x+  3 root       wheel  96 2018-07-31 02:09 app2.app/
drwxr-xr-x   3 root       wheel  96 2018-07-10 12:15 app3.app/
drwxr-xr-x   3 mr_bean    wheel  96 2018-08-16 07:09 app4.app/
drwxr-xr-x   3 root       wheel  96 2018-07-10 20:08 app5.app/


Then, &lt;ExternalHDD path&gt;/applications_2/ contains: 

drwxr-xr-x   3 root       wheel  96 2018-06-16 09:18 app6.app/ 
drwxr-xr-x+  3 root       wheel  96 2018-07-31 02:09 app7.app/
drwxr-xr-x   3 root       wheel  96 2018-07-10 12:15 app8.app/
drwxr-xr-x   3 mr_bean    wheel  96 2018-08-16 07:09 app9.app/
drwxr-xr-x   3 root       wheel  96 2018-07-10 20:08 app10.app/


In your terminal, for app6, 7, 8, 9, 10: 

ln -s &lt;ExternalHDD path&gt;/applications_2/app&lt;n&gt;.app &lt;InternalSSD path/Applications/.


Finally: 

ls -al &lt;InternalSDD path&gt;/Applications 


should return:

drwxr-xr-x   3 root       wheel  96 2018-06-16 09:18 app1.app/ 
drwxr-xr-x+  3 root       wheel  96 2018-07-31 02:09 app2.app/
drwxr-xr-x   3 root       wheel  96 2018-07-10 12:15 app3.app/
drwxr-xr-x   3 mr_bean    wheel  96 2018-08-16 07:09 app4.app/
drwxr-xr-x   3 root       wheel  96 2018-07-10 20:08 app5.app/
lrwxr-xr-x   3 root       wheel  96 2018-06-16 09:18 app6.app/ -&gt; &lt;ExternalHDD path&gt;/application_2/app6.app 
lrwxr-xr-x+  3 root       wheel  96 2018-07-31 02:09 app7.app/ -&gt; &lt;ExternalHDD path&gt;/application_2/app7.app 
lrwxr-xr-x   3 root       wheel  96 2018-07-10 12:15 app8.app/ -&gt; &lt;ExternalHDD path&gt;/application_2/app8.app 
lrwxr-xr-x   3 mr_bean    wheel  96 2018-08-16 07:09 app9.app/ -&gt; &lt;ExternalHDD path&gt;/application_2/app9.app 
lrwxr-xr-x   3 root       wheel  96 2018-07-10 20:08 app10.app/ -&gt; &lt;ExternalHDD path&gt;/application_2/app10.app 


With this method, you don't modify your base system, don't break any path in your OS and symlink the content of your new app folder to your old one.