Is it safe to take 2 overactive bladder medications Detrol (tolterodine) LA 4mg with Oxybutynin Er 10 mg...

Not a good idea. They are the same class of medicine (antocholinergic) and both work in pretty much the same way, by reducing spasm of the muscle that contracts to push urine out of the bladder. Taken in excess they can actually cause incontenence by paralysis of that muscle (overflow incontenence). If the meds are not working you should see a urologist. You can have systemic side effects with too much meds.
Drug interactions. There is some potential for interactions. See: http://www.drugs.com/interactions-check.php?drug_list=1768-0,2215-1467 Please ask your personal physician for best recommendation. Take care.