Adc/sup are probably overtuned yes, but that's a different statement from implying 1 solo laner = 2 duo laners in an ideally balanced perfectly played scenario.
If both people are perfectly csing, the solo laner and adc are going to have equal gold, while the support is going to have their share of ambient gold, plus whatever they got from their support item, so gold advantage is to the duo. The duo ends up with 30.4% more experience as well, although what matters is levels, so that xp advantage may still leave them a level or two behind.
Reddit tells me the average gold value of a level up is 532.91, so it comes to whether the gold advantage the duo has outweighs the level disadvantage they have. For them both being 1 level down, that means the supports gold would need to be around 1.1k at that point, or 2.2k if they're both two levels down.
Looking through most of my match history, I generally make at least 7k on supports by 30 minutes, so it seems fairly safe to say that the gold advantage of the duo should pretty much always outweigh the level disadvantage, assuming all players are playing perfectly and directly translating their stats into combat effectiveness.
In practice of course that's not the case, and the relative value of xp versus gold is quite champ dependent (supports often chosen precisely because they don't value gold as much, and adcs don't mind the support xp leech since they value gold more) which complicates the analysis somewhat. Overall game tuning doesn't change based on who is picked where though, so in principle I don't think 1 solo laner should inherently be equal to 2 duo laners just on a numbers basis. (I think it's pretty clear that janna mid shouldn't be on equal footing with trist adc+talon support for example.)
Don't get me wrong though, they probably do need to tune down marksman/many conventional supports somewhat, just that being able to 1v2 in perfect play only makes sense for fairly significant leads, and it'd take some pretty harsh penalties to xp sharing to change that.