Hello, I have 25mg viagra (sildenafil) tablets but they not doing much. Is it ok for me to take 2x tablets to make it 50 mg?

Dose upto 100 mg. As long as you do not have any contraindication- read the drug info leaflet or google for side effects. Start with 50 mg & gradually increase upto 100 mg. If not effecive see a doctor.