Where is the best place to fit the O2 sensor. 

My '04 V11 LeMans Rosso Corsa does not have an O2 sensor at the moment but it does have two blanked off ports on the downpipes  by the front crossover.

I have a spare standard rear crossover that has one port on the left fitted with an O2 sensor, I assume there is a catyliser inside this crossover.

If running double sensors with the MyEcu should I fit them to the two foward ports or weld in a second port to the right of my spare crossover. Alternatively should I weld in two ports to my existing setup - if so where?

The standard placement on those bike fitted with a gas sensor is on the crossover just before the mufflers.
This is a good spot for single sensors.

Generally sensors position is not too critical as they tend to have heaters builtin but one thing to avoid is placement too close to the exhaust outlet as too much heat is not good.

Unless you are going for race track performance, one sensor is enough.

Here is the link showing where I put my O2 sensors on my '97 Sport1100i.  I had to weld in the bungs myself.  I agree with Cliff that only one O2 sensor is really necessary.
https://picasaweb.google.com/106090307548421468637/MotoGuzziProject?authkey=Gv1s...
I hope this helps.  They are angled upwards at about 30degrees to avoid condensation problems.