There is normally a primary account holder then a secondary card is issued. The primary is the one who has control.
I'm not referring to an account where there is a "convenience signer." I'm referring to an account where two people initially apply together and the two people are jointly and severally liable on the account. They are equals.
If my husband and I applied for a credit card account and were granted the account, I'd be plenty PO'd if, simply because he's listed first on the account, he requested to increase the line (behind my back) and this was approved without my permission, making me liable for an larger debt that I did not agree to!