Pyro as Bill said:
MS and Nintendo don't need to co-produce a console to achieve both of their aims.

MS would much rather give a slice to Nintendo than Sony/Apple/Google.

All MS/Nintendo have to do now is license Switch to dock to the Nextbox.

This is a good point.  I keep wondering how this helps their console, but really they are looking to help the PC platform.  All of this cross platform stuff makes the PC more attractive as a universal platform compared to a mobile device.  It will be interesting to see if they start doing cross play on games that don't translate to mobile very well.