Is it safe to take effexor (venlafaxine) xr during the day and seroquel at night for sleep? is there still a risk of getting qt prolongation?

Yes and Yes.: Using Seroquel & Effexor can increase the risk of this rare side effect. You may be more susceptible if you have a heart condition called congenital long QT syndrome, other cardiac diseases, conduction abnormalities, or electrolyte problem (e.g., magnesium or potassium loss due to severe or prolonged diarrhea / vomiting). However, risk is small (RARE = 0.1%) & many pts can & do take both safely.