Rugby Question

Passes do not have to be two handed , a lot of one handed passes happen
[ definition of a forward pass has been discussed a lot in the media this season]

an american football is smaller

I remember Gavin Henson throwing a pass quarteback style across the width of the pitch

p.s. went to youtube to look for that pass , came across this clip which has numerous one handed passes , sadly not Henson's

As Traeth says you can pass one handed but its a more risky pass as you have less control, trying to pass a ball backwards across the width of a pitch is also very risky due to the risk of an interception.
In fact it's very rare you see long passes in rugby for that reason, strength in numbers is usually the order of the day with short passing so any mistakes or mishaps can be mopped up.
You do see plenty of one handed short passes though when a player is trying to release the ball quickly to let a team mate in when on the offensive.