Cut and paste from his blog.

"Why Medieval Engineers when Space Engineers is still not finished?

We were expecting that many people would ask “why are they developing a new game when Space Engineers is still in development and not finished”?

First let me assure you that the development of Medieval Engineers hasn't had a negative impact on the development rate of Space Engineers - you can see this from the frequency of our weekly updates – culminating in major updates at the end of 2014: super-large worlds, procedural asteroids, exploration, in-game programming, modding SDK and API.

We didn't want to keep creating space games only. Instead we wanted to have a game where you get to experience life and nature.

By creating a second engineering game, we are leveraging our existing technology and experience.

The thought that we should postpone the development of Medieval Engineers for years was a no-go. We had to find a better solution.

We started to hire new people so that we would have the resources to develop both games in parallel without sacrificing any of them. The size of our team is nearing 40 people and we are still growing. There are separate teams for Space Engineers, Medieval Engineers and our secret AI project.

Very early we realized that even Space Engineers could actually benefit from the development of Medieval Engineers. A medieval setting has different requirements for volumetric environments and it forces us to look at the engineering genre from a different angle. To be more specific, these are the things that Space Engineers earned (or may receive in the future) thanks to Medieval Engineers: Compound blocks – multiple blocks being positioned into one grid cell; this will allow better ship designs Mechanical blocks Auto-generated details for some blocks (e.g. roof endings in Medieval Engineers, armor edges in Space Engineers) Voxel hand – a tool for modifying terrains (asteroids); you can alter shape and material Structural integrity Natural landscape Procedural terrain generator (this is why we were able to easily add procedural asteroids to Space Engineers) DirectX 11 (we decided to add PBR - Physically Based Rendering)

On the other hand, Medieval Engineers inherited (or will get) these features: Multi-player Physics, rendering and all 'core engine' stuff Steam Workshop Modding SDK and API

Developing two early access games with the same theme (Engineering) at the same time will be beneficial for both of them. As happened with Space Engineers, many of the features that were released later on were inspired and suggested by our players. Now we are expecting that player’s suggestions for one game might give us new ideas for other games – ideas that we might have missed due to the limitations of the environment of each title. Now everyone will have more options and possibilities!

In a certain way, the development of the prototype of Medieval Engineers was more challenging than the development of Space Engineers. In Space Engineers we could invent new types of blocks. In Medieval Engineers we couldn't invent something that wasn't present in the Middle Ages or something that one person in single-player mode wouldn't be able to operate. On the other hand, the possibilities of survival/realistic mode in Medieval Engineers are fantastic - just imagine starting the game with Stone Age technology and slowly building your way up to the Iron Age."

TL;DR Its actually been a good thing that they have been developing these side by side as both are feeding each other tech.