I am planning to install a pair of fanfare horns on my fasty. Are there any electrical mods required. ie new relay, bigger wires etc. I imagine that two horns draw more current than one, the question is, can the stock system cope with it?

you'll need a relay

Doe sanybody have more details, size etc. I cannot see on the horns or box how much current they draw or what the power rating is. Anybody know?

have you got a spare battery?? multimeter? some loose wiring hanging round???
then connect up one horn to battery and multimeter to measure the current draw, double it and that will be the current draw for both horns.