I am not aware of those two causing any different-day interactions. Do not use them together, however. Like many other GABA-affecting drugs, both GBL and GHB have respiratory-depressing effects. This is how most drug-interaction overdose deaths are occurring these days. An opioid combined with a GABA-affecting drug (often a benzodiazepine). But as long as you space them out 24-hours at the least (48 would be better just to be safe) it should be no less safe than either drug on their own.
