Info -
Yep, that works fine.
So does postUpdate with just a state, that’s a surprise.
mySwitch.postUpdate(otherSwitch.state)
Numbers work
myNumber.sendCommand(otherNumber.state)
Strings don’t work
myString.sendCommand(otherString.state)
The reason is described in the GitHub issue:
The postUpdate method is provided with a State as parameter while the sendCommand method is provided with a Command as parameter (a Command object is not a State object).
Addtiotinally the sendCommand is provided with a String or a Number as parameter but not with an OnOffType or any other State. The example using a Number item is working because it carries a DecimalType which extends Number - it will be converted internal accordingly.
To be honest that cannot result from a “follow” profile which only forwards a state sent to one item to another item.
Well i can reproduce this anytime you like for debuging.
And as well I can do it without loop by the rule in OH or NodeRed.
So yes follow profile causes loop with tasmota devices
To be fair, it’s TWO follows that make the loop. It’s what you needed for your desired action, but in this case cannot be used.
yes, I’m quite sure what is causing the loop, but as some people said it’s working, I’ve tried and failed 
But I think issue is not really in two follows, but in fact that tasmota besides cmnd, sends update stat as well, which trigger another follow.
(if) it won’t then follow will work as expected.
I think for the string you should do myString.sendCommand(otherString.state.toString)