Medications like flomax is mainly given to improve the urine flow. Its effect on erectile function is still controversial - some studies reporting no effect at all & some showing negative impact on the erectile function.
As for practical purposes it is considered not to affect the sexual function. It definitely causes retrograde ejaculation though which is a different equation.
If any patient on drugs like flomax develops erectile dysfunction - we prefer to give one of the drugs - Viagra, cialis or levitra , provided there are no contraindications for its use.
There are some newer data which shows that cialis alone can benefit both the urine function as well as sexual function. Although not fully proved it can be tried alone in your case after discussing it with your doctor.
As far as twice daily dosing of flomax - if it is 0.2 - its OK; but if it is 0.4 then it might be time for you to change to a different drug. Double dosing will increase the side effect profile without giving equivalent benefit.
Silodosin is a better drug to improve urine flow in prostate enlargement.
I guess you can discuss these options with your Doctor and take corrective measures.
I hope this helps you.
If you have any questions , please let me know.