Not sure. Weirdly, there is almost an identical thread posted less than a hour before your's - only the amounts are slightly larger - they were asking if it is safe to take 4mg lorazepam and 300mg tramadol 6 hrs after taking dextroamphetamine. They don't have any replies yet either unfortunately.
I would say that if you take the lorazepam every night then it shouldn't be too much of an issue, so the main issue here is whether it's been long enough since the dex. I'm assuming the risk you are worried about is serotonin syndrome? If the dexedrine is immediate release, the effects generally last from 4-8 hrs. I am no expert, but I would suggest waiting a bit longer if you can.