Is it good practise to mix double and triple zeta basis sets?
It is perfectly fine, and actually quite common, to use big basis sets for the most important atoms (perhaps at the active site of the chemical reaction) and a smaller basis set for the surrounding hydrogen atoms (for example).

The only problem you might face would be, that this complicates the basis set extrapolation to get closer to the complete basis set limit. Such an extrapolation works best if you are, for example, extrapolating from 2Z &amp; 3Z to infinity (not a mixed extrapolation such as 2Z/3Z &amp; 3Z/4Z to infinity). In fact you won't be able to extrapolate because you likely won't be doing calculations with a 1Z basis set, and you won't be able to do a 3Z/4Z mix (because if you could do that, then you wouldn't need to mix 2Z into your 3Z calculation!).

I do want to point out that if you need to compute "extremely accurate single point energies" then even if you could use 3Z on the whole molecule, it would not likely even get you to 1 kcal/mol precision for energy differences. So as long as "extremely accurate" means in your context "+/- 10 kcal/mol for energy differences", you are doing everything just fine (and don't need a basis set extrapolation anyway). If you are aiming for better accuracy than that, you'll probably need a basis set extrapolation (meaning you want all atoms represented at 3Z level). 



In the following reference a "mixed" basis set was used;
in this paper TZP (3Z) is used for the atoms Fe, S, and Mo while SVP (2Z) is used for C, H, O, and N:

Li, Z.; Li, J.; Dattani, N. S.; Umrigar, C. J.; Chan, G. K. The electronic complexity of the ground-state of the FeMo cofactor of nitrogenase as relevant to quantum simulations. J. Chem. Phys. 2019, 150 (2), 024302. DOI: 10.1063/1.5063376.

Since the molecule is FeMoco (an iron-sulfur complex) it is likely that the Fe and S are very important (so they have a big basis set). It is also believed that the molybdenum (Mo) is important, because why else would a biological molecule have Mo in it, so it has a big basis set too. The rest have a small basis set.