Are the beneficial chemical constituents in green tea and black tea identical, or are some unique within each? Would it be beneficial to alternately drink a glass of green tea, then black, or simply make tea using teabags of both types at once, as green tea has such a mild taste, to get exposure to both types in equal measure, or is there a better ratio?

Your intuition is spot on here that both types of tea, having unique substances and both being beneficial, have these effects owing to their unique makeup. So your strategy is quite sound, that the best of both worlds is to actually use both types of tea on a regular basis and thereby gain the benefits of exposure to both sets of beneficial molecules they contain. Your strategy of alternating to create an equal exposure is perfectly satisfactory. The exact ratio to be optimum will vary from person to person depending on their individual makeup and vulnerabilities for which the constituents of the respective forms of tea will have a benefit, and that cannot be predicted readily or determined in any consistent way. In any event, what you are talking about here is having a significant exposure to the beneficial contents of both types of tea, so you are essentially creating a foolproof strategy because anyone like yourself who adopts this regimen will get what they need whether it is predominantly one type that is needed and much less in the case of the other, they will still have enough with a regular alternating regimen of multiple glasses during each day. So this is something that could benefit most human beings.