The connected watershed can be seen on WaterwayMap.org, on the “Natural Waterway (excl. canals)” map view, which is aimed to be “naturally flowing waterways”. (wwm.org intro post)
The current tag filter for calculating this is: waterway∈dam,weir,lock_gate,sluice_gate,security_lock,fairway,dock,boatyard,fuel,riverbank,pond,check_dam,turning_point,water_point,safe_water→F; waterway∈ditch,drain,tidal_channel→F; waterway=canal∧lock∈yes,disused→T; waterway=spillway∧area=yes→F; waterway=canal∧usage=spillway→T; waterway=canal∧usage∈headrace,tailrace→T; waterway=canal→F; waterway→T; F using the osm-lump-ways tag filtering system¹
It’s the waterway=stream tag on w413263368 which causes these 2 watersheds to be joined.
Initially I thought it was a mapping mistake, but now I think it’s a regular river bifurcation. Changing this from waterway=stream to something else might be a good idea. But I think any map of “connected natural water flows” will have to include join the Garonne & Ebro like this. 
¹ Yes this is a new tag filtering system. 
No, I don’t want to implemented SQL