While it is not possible to combine two droplets directly you could run a distributed filesystem between multiple droplets to use their storage space as a single folder. This tutorial will walk you through setting up a GlusterFS distributed filesystem cluster on your droplets.

In this tutorial when creating the cluster it uses an argument “replica 2” which tells the cluster to keep two copies of each file. If you are not concerned with redundancy and simply want to pool storage you should not include the replica option. This will then allow you to use the full space available between your droplets.