I am having issues with constipation cause by medicine topomax 100mg and neurotryptaline 10mg. may i use miralax twice in one day?

Dose-dependent: The more MiraLAX (polyethylene glycol) you take the faster it works. As a once over-the-counter dosed laxative you may not see results for 3-4 days, but as a multi-dosed colonoscopy preparation (when used under doctor's advice) expect laxation within hours. As with any laxative however understand its mechanism of action, its risks, treatment goals. Also, make sure you are not missing obstructing.