AFAIK, the only way to use a single disk on two machines would be as a network drive. This can easily be done by connecting it to one of the Macs and enabling File Sharing in System Preferences > Sharing. It will then show up in the sidebar of Finder windows on the other Mac. With gigabit ethernet you should get around 100MB/sec throughput like this, which is a lot slower than some directly connected drives (for example, I have an external USB SSD that gets 400MB/sec).
There will be a small performance hit on the machine that does the sharing, probably not much of an issue in most cases. And, obviously, the machine that is connected to the disk will have to be turned on and connected to ethernet for this to work.
If you don't want to go that route, you will need a dedicated fileserver of some sort, such as a NAS. There are some relatively inexpensive products for that, but last time I looked their performance was not very impressive. You don't really give any detail on what kind of storage device you are using, if it has ethernet ports then maybe it actually is a NAS?
The newer Macs all have 802.11ac wifi that is pretty fast. I use that on my MacBook Air but only get around 60MB/sec throughput over wifi, as opposed to 100MB/sec on ethernet.
