Best way as the max profit from single thing, if not obvious. And unfortunately, requirements add no play value.
The value would be there if the quest for example had multiple ways to be completed, and using science was one of those.
Requirements add play value as you do profit more if the skill is commonly needed, not just optional and you simply get more out of it by having the certain skill. What you are pointing to, is just simple quest quality, diversity, complexity, call it what you want, that of course ultimately adds more play value as there are more options (if quest has high play value, you get more joy, fun out of it), in that case quest itself brings more to the game, but having those options will not add extra weight on a certain skill - it'll not introduce new mechanics and new something for the skill at all, it will be just as profitable as before. It's still one requirement for science, repair etc. etc. per quest even if it's possible to complete to complete it in many ways.
I'm talking about science, repair in general, what is possible with those particular skills, what game mechanics are tied to them, what you get out of them by having them, the more ways how the skills can be used are added to the game, the more play value comes along with it, as previously explained requirements and necessity also does the trick in a way, maybe not as someone wish to since that is partially a matter of opinion, consensus might be split on whether those skills have their "right amount" of purpose and part in this game at all.
It's no different than quest being available for all, just more annoying.
Here's the thing, i just simply disagree.
For example, force fields in Ares and Sierra Army Depot are a new mechanic as you need to bypass them to continue on with the dungeon, the skill has one more usage and it adds play value, but then again since it's not possible to continue on without repair or science, those are the same requirements as in quests, you just experience the requirement differently.
Tell me is it annoying to need science or repair for you to be able to craft an item?
Everything Avv suggested can be done without merging skills, now tell me why the hell someone needs to merge them? What is this, blindly following some trend?