How to drop in using echo plus to echo dot using voice command

[Fred 21]

How can I use a voice command to go from echo plus to echo dot (or vice versea) ?  All are enabled.  I can go from my tablet and click the icon at bottom for a drop in (that works), but I also want to do a drop in with the two devices using just my voice command.  Please give details as I'm not big on tech.   Thank you for your help