I've been taking Effexor (venlafaxine) 75mg for almost 5 months, can I start taking two a day if I feel my dose isn't working?

Yes ,but. Yes, you may take two a day but be sure you see your doctor for blood pressure monitoring.This drug has norepin., at higher dosage, blood pressure needs to be monitor.Also make sure that you have blood test done for liver and kidney function tests because if there are any renal or hepatic impairment, then dosage needs to be reduce 25-50 % depend on severity.
Agree With Dr Bey. I would only add that you should always consult with the prescribing physician before changing any medication dose. There may be a reason you are at your current dose. It would depend upon other medications you are taking, certain medical conditions, and your general state of health.
Yes. Below 150 mg Effexor (venlafaxine) acts like an ssris it's only when you get to 150 mg that it acts like a dual acting antidepressant and raises norepinephrine as well.