A dev team got to large and have to split in 2. How do you approach/organise this?

If possible, try to assign areas of responsibility so you avoid to much collisions between teams.

Setup some common regular meetings to sync what you are doing and maybe have some rotation of people over time to avoid silos where you only ever communicate within the teams.

But it will also depend a lot on what kind of development your doing.

The modern approach is to split them up so that each dev team has a maximum of diversity with no regard to nonsense like skills.

If it is web development, then Infrastructure should reflect it by having independent areas where two teams could develop with minimal impact on each other.

Talk with them. Team splitting should be a last resort, so try to minimize damage by trying to find out which team members can work together and why - and how their skill composition is.

It won't be possible to fulfill everyones wishes, of course - but include them all if possible.

Otherwise you'll most likely end up with a "great management tore us apart, this place is bonkers" attitude.

Scissors and duct tape