I managed to put two stacks of the same tokens on the board (could’ve been three!). How?
Use Willbreaker (whenever you cast x, gain control of Greg’s first creature) and Dowsing Dagger (give Greg a token).
Now, cast Dagger. When the token gets stolen, cast another Dagger. Now, Greg has the token as well (again). When the first token returns at the end, it doesn’t stack. If you do it often enough, Greg will have three useless defenders. Profit (better pack flyers)
//Edited Title - Brigby
Interesting follow-up: I tested with Limits of Solidarity, Malevolent Whispers and Enthralling Victor.
Same setup for Limits of Solidarity: cast one Dowsing Dagger, then Limits to grab the plant tokens, then another Dowsing Dagger. In this case, the plant tokens did properly stack once my turn was finished, creating a stack of four (two direct cast plus the two stolen).
On my next turn, I used Enthralling Victor on the stack of four, then cast another Dowsing Dagger. In that case, the stack of four came to my side, then the stack of two was generated (all correct thus far). However, when the stack of four went back to the opponent at the end of my turn, it both reinforced the stack of two (now a stack of six) and created a new stack of four in the second creature slot.
In a separate match, Malevolent Whispers worked the same as Limits of Solidarity, correctly stacking the tokens after the end of the turn.
Limited sample, but thus far it seems like it’s an issue with the creature effect (Enthralling and Willbreaker) than a broader issue with the tokens not restacking in general.
I’ve had some fun with this one too.
You’re spot-on with the plant tokens not stacking.
I can go a step further too that resulted in my NOT running Dowsing Dagger along with Willbreaker anymore.
I have a Bolas I build named “Git Off My Lawn!” that steals creatures and kills them in all sorts of ways (Thud, Costly Plunder, etc.). Very fun build…but…
I had a Willbreaker and Captivating Crew out both stealing creatures.
If you’ve got 2 of Greg’s stolen creatures (really easy when you’ve got Dowsing Daggers) and he has 1 still under his control.
We’ve established that the plants did not always stack correctly.
I have Willbreaker in the bottom slot, CC in the 2nd slot, plant tokens in the 3rd and 4th slot.
Note: Whether these are 2 autonomous plant tokens eating up the 3rd and 4th slot don’t seem to matter.
Greg has (insert any other creature here) in his 1st slot.
I throw another Dowsing Dagger.
Greg now has more plant tokens in the 1st slot and (any creature) in the 2nd.
After combat resolution (I’ve done 0 damage this round), the game soft-locks when the 2 stolen critters in my control return to Greg’s control…because Greg can’t have 4 critters on the battlefield.
They might have fixed that bug with 4 possible critters on Greg’s field, but I haven’t run Dowsing Dagger in this build since before M19 hit Booster Crafting.
So some other fun effects of swapping creatures…
If your opponent has two creatures out, you can permanently steal one using a temporary steal card (Limits of Solidarity, Malevolent Whispers, Act of Treason, etc.) to steal a creature, then immediately cast Switcheroo. Switcheroo effectively makes the game “forget” that it’s supposed to send back the creature to Greg because you already sent the stolen one back yourself. It’s an expensive and roundabout way of doing things compared to just casting In Bolas’s Clutches, but it’s still effective.
Also fun:
I have one creature on the board. I cast In Bolas’s Clutches to steal Greg’s Siege-Gang Commander. The stolen Commander generates the goblin tokens, so now I have three creatures. (Debate amongst yourself if Clutches should trigger an Enter the Battlefield effect, but that’s just how it works.)
Immediately after in the same turn, I play Dowsing Dagger to generate Plant tokens on Greg’s side, then immediately after that play Switcheroo to swap my first creature with the Plant tokens. Then all hell breaks loose…
Switcheroo causes my Siege-Gang Commander to be destroyed because there is a bug with Switcheroo that destroys the stolen creature if your creature board is full. (Bug #1)
However, it’s not actually destroyed, but instead sent to my hand. Why? In Bolas’s Clutches has the clause that if the stolen creature is destroyed while the support is not in play, then it’s sent back to your hand instead of the graveyard. But In Bolas’s Clutches was in play because it was just cast two cards prior (effectively resolved, since I had the Commander and the tokens) and hadn’t done anything else. So the game didn’t recognize that the support was actually in play. (Bug #2)
Wait, if Switcheroo destroys the creature it claimed due to Bug #1, why did it destroy my Siege-Gang Commander (stolen by Clutches) and not the Plant tokens (taken by Switcheroo)? No idea. Speculating that it’s because the game looked up the first card that switched sides, not expecting that someone would steal two cards in the same turn. But that’s just a random guess. (Bug #3)
All’s well, I cast the Commander next turn and went on my way. But it was a real bizarre moment when it all happened. I did already submit this as a support ticket and they’ve acknowledged the known bugs, just thought I’d share here as well 
Hi Everyone, the team has confirmed that they are now aware of this issue. I’ll go ahead and add this to the bug list.