One major concern is connecting the grounds of the chargers to 2 different voltage points. If the charger outputs are not completely "floating", meaning isolated from ground, that would create a short circuit of sorts. Not good, would not recommend trying it. Even with floating outputs there is the possibility of any control noise from one charger interfering with the other.

If done one charger/cell at a time balance shouldn't be an issue as long as the charger is correctly charging a single cell to 4.2V. I experimented with charging and discharging individual cells like this before I had balancing chargers. I used the technique to balance packs and to experiment with cell balance and such as I was learning about charging and balancing lipos. And then learning about and testing balance chargers.

If I may ask I'm curious why you want to try this?