Our thanks to you for explaining about Ekadasi observance during festival days. While there are only two people in the house while observing ‘nirjala upavasam’ (fasting without even taking water) can we offer fruits to Perumal during Tiruvaradhanam, is this applicable for Ekadasi that occurs during the festival?

If Ekadasi occurs during a festival and the food offered cannot be partaken, hence one can offer fruits.
As preached by Perumal, He accepts leaves, fruits, flowers, and water; fruits can be offered as such.
If Ekadasi falls on festival days, if there is no use for the offerings to Perumal, fruits can be offered instead. But if Ekadasi occurs during important festivals like Sri Jayanti, one has to offer special delicacies meant for Perumal even though one is fasting and partakes them the next day after the completion of Ekadasi vratam.