Open the "gPodder Podcast-Client" on your PC or MAC.
Upload your podcasts and if it's asked, enter your username and password. The server will create a new legacy device for you, if no device exists.
Click on your "Subscriptions"- or "Devices"-link.
Now you can see your devices, click on one of them.

You are redirected to another page.
On the bottom of the page there's a field named "Edit"

Here you can edit the name, the type and the uid. But pay attention if you edit the uid! If you enter a false one your subscriptions won't be synchronize with your device anymore.
Choose a device on your "Subscriptions"- or "Device"-screen and click on it.

Go to the field "Synchronize", choose your desired device and click on the "OK"-button.

Now the two devices are synchronized. If this isn't enough, you can synchronize your device with one or more devices!
Click on your "Subscriptions"- or "Devices"-link.
Choose a device on which you would like to stop the synchronisation and click on it.

Go to the field "Synchronize" and click on "Stop synchronisation for your device"

Now the two devices are unsynchronized again.
Click on your "Subscriptions"-link.
Choose the podcast which you would like to copy on another device and click on it.

Below the field "Devices" you see the link "Subscribe to ... on other devices" click on it.

On the next page you can choose your desired device on which you want to copy the subscription.
After hitting the "OK"-button, the subscription has been copied.
There are different methods to do that:

Below the field "Devices" you see on which devices you've subscribed the podcast. Now select the device on which the podcast schould be unsubscribed and click on the red/orange-minus on the right.

You are redirected to the same page, now the device from which you unsubscribed the podcast shouldn't exist amongst "Devices" anymore.