I wonder how common is it for a starship to make their "Senior Science Officer" a two person team.

Clearly many captains don't see filling the role as a priority: None of the Enterprise D, Voyager, or (until this episode) the Cerritos had a senior science officer posted to the bridge. This makes a fair bit of sense on the grounds that "science" is an extraordinarily broad field and most of the practical, problem-solving sciencing we see tends to fall under the umbrella of engineering, so subject specialists and engineers wind up carrying the load as appropriate. As they should! No science officer can possibly hope to be comparably well versed in any given subject than a more junior officer who happens to specialize in it.

Therefore, although scientific acumen is obviously useful, as is getting as much scientific acumen as possible onto the bridge to quickly react to whatever weird shit a ship encounters, the larger part of the job is going to come down to synthesizing the larger knowledge base of the ship's contingent of scientists into an actionable answer. The Senior Science Officer should be asking themselves not just "what do I know about this", but "who else on the ship knows more about this" and, in a pinch, "which of these different ideas are we actually going to try."

Having two people in the role is beneficial for getting more scientific knowledge on the spot and ready to be used, especially if the two people involved work well together, but it's a potential liability in that final point where two people can reasonably disagree, but someone is going to have to make a call on what the best option is. In many cases that person is the captain, but when time is scarce and the choice is between things the captain doesn't understand, the choice is really going to come down to the science officer. And what happens when the two science officers disagree?

In this case, I think the correct choice between our two science heroes would be Tendi. She's (generally) good with other people, she has actual command experience (in combat situations, no less), she knows the bridge officers better, and they are more familiar with her. Further, T'Lyn is nominally a temporary posting who doesn't seem to view herself as a serious candidate for the role. All the conventional decision-aiding factors seem to favor Tendi, and we all know she's qualified.

Which leaves me wondering why this was a particularly difficult decision for Freeman, and why Data recommended this seemingly unconventional solution. Heck, Data barely even saw these two officers working together, as the two of them spent nearly the entire time working independently.

So maybe having multiple "senior" science officers is a normal state of affairs, and the expectation is that the captain will ultimately be able to resolve any final-stage disputes without needing an explicit head of the science division? That's plausible if potential awkward, and there do seem to be plenty of consoles at the back of the bridge for multiple science specialists to be sitting in.