If you want to have multiple ELEMENTICE™ devices at one event, each with a different watermark there are two ways to achieve this...
One way is to create separate Events for each device and use a common Gallery Card / common ID method... but that can get complicated depending on your ID source and number of devices etc.
Another way is to enable "Device Specific Watermarks" for the Event and set the watermark you want for each device individually. If you set a watermark on the Device Page and then go to the Event page and select "Yes" allow - That will allow the Device watermark to override the Event Watermark. You can also set an Event Watermark which will be applied to images from any device which does not have it's own special watermark set.
See here for more on enabling Device Specific Watermarks.