Idea no.... 4 now? A shield where you can mount two tinyduino shields next to each other, for example a radio and bluetooth module next to each other. I can't imagine it being too difficult to produce...

Should solve some signal problems

OH man, now that I want to tell you about it I can't find the link. But yes! we have a tiny circuit ribbon cable shield where you can have to separate stacks of boards connected together.

I would like to have a single board to mount two shields side by side horizontally, that is good for wearable projects I think.

Just like this one, there is horizontal version and vertical version.
https://www.microduino.cc/module/view?id=53eb213a3f6ada00004fb0f4

With the ribbon cable shield only able to connect two boards up and down and not fix in position.

Similar to the side-by-side shield, we have a back-to-back style breakout board that is just about ready for production, although we don't have a planned release date for it.