If an individual is the owner of a small company with a 401k plan and desires to set up a second company, will both firms need to fall under the same retirement plan due to affiliation? Does the testing on the plans include both firms ?

Could he set up a Solo 401k in his new firm (single employee) and put enough money to get him to the maximum of 49k minus the salary deferral he has saved in his 401k firm?

What are the rules he must follow?