They did compete alongside each other - under iF20 rules in Europe.
Sorry... my reference was for within the USA. NACRA provided the I-20 here with a larger sailplan. That made US competition against the Fox as an F20 impossible.
I'm sure that if F-20 competition were desired here either the I-20s could have produced a reduced sail plan for it or the Fox could have had an increased sail plan under a modified F-20 rule set.
