You have not indicated the industry to which you belong, which is very important. Anyhow, following are my suggestions:
a) One increment can be for doing routine work, while the other one could be linked to the achievement in KRA.
b) You need to define KRA for every employee.
c) Also, define the range of the percentage of increment, i.e., say between 8% and 15%. Otherwise, many employees will demand a 50% hike.
d) As explained by Jane, be careful while implementing the policy. It is not easy to withdraw or modify it.
If possible, instead of giving two increments, give only one increment linked to performance in KRA. This is more practical. You can fix the range from 8% to 25% so that the extraordinary performer will get 25%, while the below-average performer will get 8%.
Regards,
Bhavan