If you have the dual tone sirens like most Fords they should work at the same time, one being marked 'high' and the other 'low'.

I think some cars have the single diapham round flat thing horn, their poo - bin it!

Get some nice new ones, i bought a new twin tone horn set from halfords a while back, they had to be wired up with a relay but that was easy enough............wait till you try'em scared the crapo outa me, they were soooo LOUD