I've been reading up on druid spells for future use and was quite interested in the crowd-control abilities of both Transmute Rock and Wall of Thorns, especially since Transmute Rock doesn't require concentration.
Both of the spells have a similar clause
Wall of Thorns (PHB, p. 287):
A creature can move through the wall, albeit slowly and painfully. For every 1 foot a creature moves through the wall, it must spend 4 feet of movement.
Transmute Rock (XGtE, p. 169):
The ground in the spell’s area becomes muddy enough that creatures can sink into it. Each foot that a creature moves through the mud costs 4 feet of movement,
So my question is, if they were cast together, would a single feet of movement costs 8 feet of movement(or possibly 16) instead of 4? One could argue that WoT mentions solid ground, but you could make the mud only 5-10 feet deep while the thorns are 10-20 feet high.
I also have a side-question to this, if they did work together, would you rule it to be difficult terrain or would there be the need for a third spell like Erupting Earth to turn the terrain difficult? Or would the mud already be considered the terrain and therefore it wouldn't be possible to actually turn the terrain any more difficult.