Can you have Toxic Spikes and Spikes at the same time?
2 layers of Toxic Spikes and 3 layers of Spikes can be laid down at once.
Source: I used them on Forretress all the time.
Yes, you can
If you lay down 2 layers of toxic spikes when the user switches out with another Pokemon
he will get badly poisoned not a normal poisoning
Badly poisoned is like the poison from "Toxic" it worsens every turn
and with Spikes you can lay them three times to inflict max damage you can from that move
if you put them 1 they will inflict 1/8 damage from the Pokemon's full hp
2 they will inflict 1/6
3 max they will inflict 1/4
and then if someone uses this against you, you can clear it using "Rapid Spin"
hope this helps!
Source: My Experience