For Toxic / Leech Seed:
The regularly increased variable N, which removes N/16 of the Pokemon's HP, is used for both of these. With Leech Seed it normally isn't increased each turn, but with Toxic in effect as well, the counter is increased twice each turn, once after each of those recurrent effects. For instance, 1/16 from Toxic and 2/16 for Leech Seed, 3/16 from Toxic then 4/16 from Leech Seed, etc. By the fifth turn of that effect, it'll 1HKO the Pokemon. However, both are reset to 1/16 and don't increase the first time you switch out, just like Toxic by itself.
The regularly increased variable N, which removes N/16 of the Pokemon's HP, is used for both of these. With Leech Seed it normally isn't increased each turn, but with Toxic in effect as well, the counter is increased twice each turn, once after each of those recurrent effects. For instance, 1/16 from Toxic and 2/16 for Leech Seed, 3/16 from Toxic then 4/16 from Leech Seed, etc. By the fifth turn of that effect, it'll 1HKO the Pokemon. However, both are reset to 1/16 and don't increase the first time you switch out, just like Toxic by itself.