Is it possible for two benders to have a child who could wield both abilities of its parents?
No. In order for an individual to hold more than one element, he/she needs Raava to hold it for him/her, as we learn in season 2, episode 8: Beginnings, Part 2
Also, we saw a couple of examples where parents from different nations had children who could only bend one element:
- Mako and Bolin's father was from the earth nation and their mother from the fire nation. Each one of them could only bend one element.
- Aang and Katara's children could only bend one element- water or air.
As Mor Zamir's answer states, no one can bend more than one element, and the only reason the Avatar can do it is because of the spirit Raava.
However, as this question speculates, it might be that certain specialised forms of bending may be possible due to having heritage of different nations; specifically that lavabending might only be possible for a child of an earthbender and a firebender (such as Bolin).
The Avatar Wiki page on lavabending has as it's first Trivia point:
Following that second link, we find this quote:
- Lava bending is a combination of earthbending and firebending.
So it would appear that lavabending likely does require a firebending and earthbending parent.
However evidence thus far has shown that only someone who is primarily an earthbender can perform lavabending (firebenders cannot do it), so it is still the case that someone with earth and fire heritage can still only be either an earthbender or a firebender.