I would eliminate the problem entirely by making the meditative concentration a linked skill....
This works, but it works differently. Let's assume that we're talking about a magical bonus to a magic skill.
If the base time factor for the skill is one minute, and I make the ritual two minutes, I have doubled the base time and get +10 on my chance of success. Each time I double the base time I get another +10, so if I make it a 32 minute ritual I'm going to get +50. This means my chance of success is 50% greater and it will always take me half an hour to perform the skill.
Instead, let's add a connected skill that bonuses my chance of success. The baseline bonus is going to be +RS/10 and it's going to take me one minute to perform; if it also takes me one minute to perform the skill, it now takes two minutes to perform both skills, and I have to roll the dice twice. If the first roll succeeds, I will add some amount of at least 1 and not greater than 10, and it will probably be on the lower end at least until I get fantastically good at it; if I manage to get my chance of success to 100%, the average bonus will be 5.5. But I can increase the ritual time of the adjunct "bonusing" skill. If I double it to two minutes, I get +10 on my chance to get a bonus of RS/10 on the next skill. I can increase the ritual time to thirty-two minutes and so get +50, but that goes to the bonus skill, which still confers only +RS/10 to the target skill. I can increase the bonus value by taking -10, which will give me +RS/5 but only +40 on the chance of success for the same time; I can increase it again to RS/2; I can even increase it to RS=SM, that if the roll is successful its value is added to the targeted skill--but now the bonus on my chance to succeed for the half hour is only +20, and there's still a 1% chance that I will roll 01, getting only a one point bonus, and probably still a significant chance that I will fail the bonus skill entirely, and thus have spent the entire half hour for no gain.
Thus it is always more effective to put the bonuses directly into the skill you want to perform rather than a secondary skill. The chief advantage of putting the bonus into a secondary skill is you can get better at that skill and use it for a broad range of other skills. The chief advantage of not putting the time into the primary skill is that you can perform the primary skill more quickly if necessary.
There is also the detail that if you are rolling for two separate skills, you've got two chances to fail and two chances to botch. You might increase your chance of success on the second skill (and so decrease your chance to botch), but you might botch on the first, or you might botch on the second.
Also, it is generally not the referee's decision which way that happens. The player/character is making a determination as to whether to limit his ability to use a particular skill by making it a skill requirement that he invest the extra time into it (which prevents him from performing the same skill the same way in less time), and so getting a greater bonus on the chance to succeed with that skill. If the player prefers, the character can create a skill that bonuses a broad range of other skills, and if he wishes he can make certain skills contingent on a successful prior use of the bonusing skill (which would give him +10 for the requirement of successful use of another skill and prevent him from performing it at all absent that success).
--M. J. Young