Sums and commutators of homogeneous locally nilpotent derivations of fiber type

Let A be a commutative associative integrally closed -algebra without zero divisors effectively graded by a lattice. We obtain a criterion of local nilpotency of the sum of two homogeneous locally nilpotent derivations (LNDs) of fiber type on A in terms of their degrees. The same problem is solved for commutators of two homogeneous LNDs.