Half speed does not stack, but damage should
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don’t combine, however. Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap.
If damage is an effect of the spell, funny things can happen. Imagine two clerics (C1 radiant, and C2 necrotic) using this spell, in partially overlapping areas. Two enemies (E1 and E2) are running past them. E2 gets the damage twice, but E1 only once, as the radiant field “protects” him when he would enter the necrotic field.
I really hope this is not intentional.
It seems it is up to us to decide what is an effect and what is not. I think if damage is not an effect, but a separate property of a spell, the game provides better simulation, while still keeping balance.
Two Dire Wolfs bite you, you get the damage twice (2x 2d6+3), but once you are prone, you won’t get proner.
Analogously, you are in the area of two Spirit Guardians. You get the damage twice, but your speed is only halved once. Makes sense.
Assume C1 has the Light domain, and he casts Wall of Fire in a 4th level slot, while C2 casts Spirit Guardians in a 3rd level slot. The enemy now takes 3d8+5d8 damage. Casting Spirit Guardians in this 4th level slot would decrease this damage to 3d8+4d8, so it is far from overpowered.
On the other hand, adding two (or more) Blesses together would be clearly overpowered, trivializing GWM for example. But Bless does not do damage, rather it is more like a condition.
You might call it interpretation, you might call it houserule, the bottom line is the game makes more sense if damage is added together, but the conditions (effects) are not.