Along the same theme as the new Group Drop In feature, which allows you to communicate through all of your Echo devices simultaneously, Amazon has also made it possible for Alexa Reminders to be heard through all devices as well. You can now default all reminders to come from all devices or specify individual reminders to be spoken from all devices.
By default, the Echo device that was used to set a reminder will be the only device that alerts you of that reminder. If you want all reminders created by voice to be spoken out through all of your devices, you can enable the “Announce on all devices” option through the Alexa app by going to Settings > Reminders. Unfortunately, if this option is disabled, there is no way to set a reminder by voice and specify that it come from all devices. Reminders set by voice will always come from one device if this setting is disabled or always come from all devices if this setting is enabled.
If you want only some reminders to come from all devices, you’ll have to manually create the reminders using the Alexa app. To do so, select “Reminders & Alarms” from the main menu in the Alexa app and select to “Add Reminder” from the top. Then, on the bottom of the new reminder screen is an “Announces from” option where there is now an “All Devices” choice. There seems to be a bug right now that prevents you from changing the “Announces from” setting of an existing reminder, so you’ll need to delete the existing reminder and recreate it from scratch if you run into that bug.
UPDATE: Fire TV devices are included when selecting “All Devices.” Thanks to Erinescence in the comments below for confirming.