Even in that case it's better to finish one thing and start another - both will be researched in same time but you can start production earlier.
In my mind that is a different flaw of the system (although a minor one if research is not supposed to be a challenge in the game).
There really is no point in splitting scientists over more than one topic in the same base.
Production, however could use something like 2 workers producing ammo, so that the ammo wouldn't run short (though there are better solutions for that).
However, to make science more exciting, assigning more scientists to same project could produce diminishing returns.
research_points_per_turn = number_of_scientists / (some_constant + number_of_scientists)
Then it would be most effective to have balanced research, and only pool scientists at one topic when really desperate about completing this one research asap.
But the way research currently works, scientists might be assigned to the active project automatically like workers are assigned to the active production (when you hire more workers).