Starting April, 5 2018 Chase no longer allows customers to open both the Plus and Premier personal credit cards. That was the easiest way to acquire Southwest Companion Pass. Anyone who either currently has a Southwest Rapid Rewards personal credit card or who has received a signup bonus on one during the last 24 months is not eligible to open a new Southwest Rapid Rewards personal credit card. In the past it was possible to open both: Southwest Airlines Rapid Rewards Premier card and Southwest Airlines Rapid Rewards Plus card, and use points from both signup bonuses to qualify for the Southwest Companion Pass.
It does not apply to the holders of Southwest Rapid Rewards Business Card:
“The product is not available to either (i) current Cardmembers of any Southwest Rapids Rewards® Credit Card, or (ii) previous Cardmembers of any Southwest Rapid Rewards Credit Card who received a new Cardmember bonus within the last 24 months. This does not apply to Cardmembers of the Southwest Rapid Rewards Business Card and Employee Credit Card products.”
So it is still possible to earn an easy companion pass with 1 personal and 1 business credit card.
For additional limitations of Chase and other Banks for the new customers see the Unofficial Limitations for New Credit Card Applications post.