The send envelope is the receive envelope is the send envelope is the receive envelope. It's the very same envelope! You can view it as one track's send or the other track's receive.
Now the thing where you want to group multiple envelopes together in order to make a single automation edit move...
This started in Protools. What this is when you think about it is a workaround for making a subgroup bus track and then just automating that single bus track. Probably because back then adding an aux bus used hardware pci card resources.
Reaper wants you to just make the subgroup track and then automate that single subgroup track.
PS. I'd like to see grouped automation envelopes in Reaper too! With routing and sends/returns being nearly free in Reaper (assuming a host computer no older than say, circa 2001) it's nearly a moot point. In fact it's kind of relaxing to be able to just route a new subgroup and automate that instead of having to keep grouped envelopes under control! But I'd still appreciate grouped envelopes one day.