There are a handful of occasions in RM2 where skill costs would be halved, doubled, or even tripled, such as for new secondary skills relative to an existing skill or creating custom professions. However, I've not found any unified rules for how to handle this. The closest is in Arms Companion (2.3.2) that mentions doubling skill costs of 3/7 to 5 and halving costs of 5 to 4/8, and an option for doubling skill costs of 4/8 to 6, and halving costs of 6 to 4/8. Based on the examples of custom professions given in that book, there are several skill costs that have been doubled from 2/6 to 4/12, and one that was doubled from 2/7 to 4/14. There are also some skill costs that were strangely "doubled" from 2/6 and 3/6 to 6.
There's also an option in RMCV (5.3) for character-specific skill costs that talks about dividing existing skill costs. One restriction is that the first cost cannot be reduced to one unless the second cost is reduced to 4 or below. However, there are many examples of 1/5 skill costs in the basic professions through RMCII.
Based on these examples, what should the threshold be for where a twice-per-level skill truly becomes a once-per-level skill when doubled? Should the threshold be based on the cost for the first skill, the second skill, or some combination of both? If a skill that has a cost of 1/5 is doubled to 2/10, what would the tripled cost be (max of 25)?