They have similar actions (well, pretty much identical actions really). There would be no point in using them both.

I use both, 1mg Propecia and 0.5mg dut...I don't care what anyone says .5 dut is not enough to block the "alleged" 90%+ DHT it claims...and believe me I have been looking at this for over 3years! The truth is you would need more like 3mg of dut ...I believe the initial studies for dut and hairloss were run at 5mg ( much more like the real dose needed ) the shitty 0.5mg was released for prostate protection which finally gained FDA approval...If you could afford 5mg then go for it but I believe the next best/cheapest option is the addition of Propecia. ...I've seen many people drop Propecia in favour of dut believing it is more effective only to see there beloved hair fall out...many go back to Propecia. ..those that have gone upto 3mg of dut have seen some pretty good results but that costs a fair amount of dosh.

Yea that sounds like a lot of cash to drop. But if taking both Dutasteride and Propecia at the same time work than it's worth it.