Nipple? Gate valve? You're running pipe rather than tubing to/from a water heater? You're a glutton for punishment.
Brass, iron, galvanized?
To tighten a close nipple the wrench goes on the fitting it screws into for tightening, not the nipple itself. That will tighten both ends as long as what the other end screws into is stationary. If not, a wrench on each thingy the nipple is screwing into. Pipe thread, being tapered, lets you crank in down more after it's tight enough to not leak. This allows turning a fitting to line up with the next link in the chain, because you never want to back up, and makes sure both ends are tight enough.