You'd need to be very mindful of your dosing, do not go over your normal dose of each individual drug. Although meth is somewhat difficult to OD on, secondary life-threatening conditions such as psychosis, rhabdo, and severe dehydration can occur if you don't "feel" the meth as strongly because of GHBs depressant effects.
Meths effects will outlast GHB. Your best result unless you monitor your doses will be the GHB wearing off and an extremely unpleasant meth "overdose" with negative bodily symptoms consistent with vasoconstriction over whelming stimulation, and anxiety like symptoms such as muscle pain and clenching jaw, profuse sweating, and paranoia/panic. Worst case life threatenint rhabdo, dehydration, or becoming detached from reality in "secondary psychosis" which can be expidated by marijuana use during the comedown can result in behavior that can put yourself and others in dangerous or life threatening circumstances, weeks long inpatient recovery, or law enforcement consequences which could destroy your reputation and social life for years.
I don't mean to scare you. Just don't take "its hard to fatally overdose on meth" to mean its peripheral effects aren't dangerous enough to kill in or harm in other ways short term. Long term meth taken in recreational doses is understood by science to be directly neurotoxic. One time likely doesn't carry noticable implications. Make it habit and you risk cognitive impairment, months of dopamine upregulation which sounds extremely bleak, and if abused daily long term can directly induce Parkinsons Disease in one not genetically vulnerable, or at an early onset.
Just watch your doses, don't get cocky, and you should be okays. Don't let it become a habit. Always remember your euphoria is artificial and not sustainable, it's just like one understands an acid trip is a temporary escape: just exploring today and resting tomorrow.
