In general, I'd say get more spoons for several reasons. First, the two sizes you have means that you will get the same number of volumes of CO2 in your beer, no matter the style. So your brewery will deliver, for instance, a stout and a saison that have the same carbonation level. And then the second, obvious reason is that you'll have a spoon sized to the new intermediate bottle size.
To decide how much priming sugar you need, you can put "priming sugar calculator" into your favorite search engine and use one of those. You will need to know how many volumes of CO2 is appropriate for the style. And you will probably need to do some math and a few conversions to get it down to a measuring spoon scale. By the way, you can type something like "2 cups in cc" into Google and get "2 US cups = 473.176473 cc". It's like magic!
As mentioned by Max, you could save on the detailed measurement by bottle by priming the entire batch at once. But that is not your question. I understand that it might be more of a pain to have an additional racking operation (which also opens up oxygenation and contamination risk), besides needing to buy or at least cleaning another vessel, etc.