How can one droplet access another and write files

Hello, My company is pushing a new service live and we want you guys to be the web hosting providers, simply because you are awesome and I have had a great experience with you so far! I will not get into small details on what does the service do but will explain it conceptually so hopefully, you will be able to tell me if something like this is doable and if it is what is the best approach for it.

Ok so… I have a root droplet in which I have my Spring boot application which users from all around the world can use. Now what I need is to get files that are being processed by the application and write them onto another Droplet accordingly (which may or may not be on the account where the root droplet is). So in a sense, I need for my root droplet to have access to child droplets with writing privileges in certain folders, of course in a secure manner.

Is something like this possible? And if it is what is the best approach?

Kind Regards, Luka Balac