Rule Clarification. 5th ed (Concentration)

This is how I'm going to handle rule questions in the  future.
Decide at the table to keep the game moving then figure it out  and post here with any house rules being available for all to see.

The only real rule issue we had at the table  in our last game was concerning casting a second spell while maintaining concentration on a previously cast spell.

The situation.
Johann had cast illusion that required concentration to maintain (The Rehmoraz.)
Next round he wanted to use another spell.

My original thought:
Johann can't cast a spell while maintaining concentration on a previous spell, he can however do simple actions like move and attack. I was wrong, Johann cast his spell  and away we went.

What the  book says: (paraphrasing)
A caster may make actions while concentrating such as moving and attacking.
A caster can't maintain two spells that require concentration.

Decision:
Go with the  game as written.
No house rule needed.

A caster may cast any spell that does not require concentration, while maintaining concentration on a previously cast spell.