When you prepare a spell, you cast it normally, but you keep its energy, which you release with your reaction when the shot occurs. To be prepared, a spell must have a casting time of 1 action, and holding on to the magic of the spell requires concentration (explained in chapter 10). If your concentration is broken, the spell dissipates without effect. For example, if you concentrate on the web spell and the magic missile ready, your web spell ends, and if you take damage before launching a magic missile with your reaction, your concentration could be broken.
–Actions in combat., Player's manual, p. 193
Unfortunately, Spiritual guardians It is a spell that requires the user to maintain concentration. So if you tried to use this sequence of events, Spiritual guardians it would end the moment you tried Ready the Thorn whip spell out.
For the sake of the argument, suppose that Thorn whip instead, it had a Release Time of "Reaction, used when a target moves away from you". This would mean that we would no longer have to use the Ready action to prepare it, which means that it is no longer necessary to maintain concentration on Thorn whip, meaning not to break the concentration in Spiritual guardians.
In this case, you could fire the second source of damage: this is the first time this turn has the creature. entry the radius of the spiritual guardians; previously, the creature started his turn inside the radio, but he did not get in during his shift. So using Thorn whip This would treat the second source of damage of that turn.
You are complicating this
You do not need to bother trying to prepare an action. The effect of Spiritual guardiansIn addition to inflicting damage, it is slowing the movement of the creatures affected by half. Most hostile creatures have a movement speed of 30-35 feet per round, which means that at half speed, a creature that comes directly from the Spiritual Guardians is probably not more than 5-10 feet away from the edge of the body. AOE. As a result, using Thorn whip in its own turn, as a normal Action, instead of trying to prepare it + Reaction, it is perfectly useful to return them to the interior of the Spiritual guardians, causing extra damage. And if they are more than 25 feet away from you (ie, Thorn whip you can not pull them inside the AOE), you can move a few feet closer before launching it. This will cause them to suffer damage by being moved within the radius and then take damage again when they start their turn.
So, in general, this method inflicts the same amount of damage as the method you are trying to use, but does not require you to use your Reaction, it does not require you to invent a false spell that is "Thorn whip, but issued as a Reaction ", and it is easier to make sense of the rules of the game.