Yes, you can assign scientists and workers to multiple projects if you so choose. That's the main difference.
Aaand that flexibility is the reason why those 2 categories were in the original Xcom too
I agree that Medics are kinda redundants, because a hospital wing is usually fully staffed, the patient will heal itself and that takes time, no matter how much doctors are involved.
Research or engineering projects, on the other hand, have so many tasks to do that having more people to help can speed up the process.
What I'd like to see is some variability in Scientists or Workers.
Perhaps they could have a small (like 5%) bonus in their expertise field, like biology, mathematics, particle physics, composite materials, system designs, weaponry... Some are better at repairing, others at making brand new stuff or pulling appart alien stuff...
Just my 2 cents