Furosemide (50mg/2 times daily) is not effective for my nephrotic syndrome. can i take it together with hydrochlorothiazide? what is a safe dose?

Yes but: The combination is often used for refractory edema however it requires very close monitoring because it can cause dangerous electrolyte imbalance. The combination if very potent. Even better is the combo of furosemide and metolazone (hydrochlorothiazide becomes ineffective when renal function deteriorates). But your doc must monitor you - you can become very ill if used without care.