Route main downspout into another?

[parakeet]

Here is a Victorian terraced house.

- The downspout from the single-storey bay gutter goes underground and out on to the street (or, it would if it wasn't currently blocked) and is old metal.
- The taller main downspout from the main roof gutter exits into the front forecourt. It's plastic and was put there during a renovation a decade ago.

I no longer like that idea of the water just sitting in the garden like that. I feel like I want it all away from the house. Especially for the main downspout, which carries more water.

Wasn't this the original idea of pipes that exit to the street? Or is this only recommended for small amounts of water?

Assuming that's a fair idea, how could I go about doing that? Can the main one be routed into the shorter one? At what point?
Since the two things are different materials, and the shorter run looks narrower, does this pose challenges? Best get a professional in?

FYI - our neighbour doesn't have a long downspout from the main roof, only the bay. Must mean it's coming either to our gutter or their other side's?

 

[Coopa]

Hi there. Take away bay pipe. Replace with upvc to make connections better fit . Then dig out old pipe and copy position into road with new pipe . Then when pipe is replaced under floor then you can get a 112 degree ‘y branch ‘ for small pipe and a 112degree pipe bend for big pipe . Cut at your desired level on small pipe then cut big pipe and connect the two . This means all water will go out of garden like before but will have main pipe water going with it Hope this helps

 

[Coopa]

Also it may not be unblocking with rods as it looks like bay pipe has more 112 degree bends under the soil so it’s not straight and won’t rod with straight poker