dxm half life is 4 hours (half-life = how much does it take for your body to eliminate half of the dose of the substance) so in 8 hours you are pretty much safe to go, LSD is a relatively weak serotonin agonist, so Serotonin Toxicity (Syndrome) has a pretty low chance of happening maybe none, I would not recommend combining the two of them tho.
so with a day between the two you are pretty much safe to go.