There are a number of reasons you should not plan on profitably harvesting hazelnuts from hazelnut truffle trees:
Because the soil conditions that are optimized for truffle production are very different from commercial hazelnut-producing farms, whatever hazelnuts that are produced on truffle trees will unlikely be of high culinary quality.
Such truffle-optimized soil conditions include loose, airy soil for the truffles, while typical hazelnut harvest is mechanized using heavy equipment that must be driven over hard, compacted soil.
The same heavy equipment will damage the delicate truffles that begin to form in the summer, which is hazelnut harvest season.
In any event, the commercial value of truffles that will be produced far outstrips any revenue you can generate from hazelnuts. Therefore, it makes sense to focus the trees on producing truffles and nothing else.