LWaB has raised a very valid point - it's pretty critical that a tandem is not too big for the captain - particularly standover height. If the captain can not standover the bike with legs spread reasonably far apart, then it makes holding the bike steady (with stoker clipped in, any panniers etc.) very tricky indeed. Obviously, there's not a lot that can be done in this case to make it safer.
If the standover is fine, then everything else is pretty much the same as a normal bike (except for stoker handlebar height which can be very tricky to get right as it is very dependant on saddle height for the captain).
Changing cranksets on a tandem can be a quite spendy experience, so explore other options first!