You do not have to flash the primary WRT160N. You can use the existing first-party firmware with it.
You would only have to flash the second WRT160N unit with DD-WRT that you want to use as the repeater bridge.
Basically, you would have
Originally:
<-------------WRT160N#1--------------> (Dead spot)
Repeater Bridge Configuration:
<-------------WRT160N#1<=====>WRT160N#2------------->
Clients in between the two units will be able to connect to either on