Development of Rear Construction: Combine Plots or Not?

Except maybe the channel connection, I don’t think it will work the way you imagine. Both electricity and water are sized for a single-family house, you can’t just connect another one like that. You won’t be allowed to do it yourself either. I would definitely not merge plots of land. If something is ever sold / inherited / used differently than currently planned, it only has disadvantages.

I also see it as a very good argument to keep the plots separate.

Nonsense. The rear building would need GFL rights from the front building anyway in case of a (re)separation if one wanted to sell separately. I stick to my position (or rather had considered it from the start): it belongs completely combined.