It would likely only be one at a time. The maximum amount that a credit card company can garnish is 25% of your net income. If a credit card company gets a judgment that permits it to apply for an order to withhold your wages, they can elect to take a maximum of 25% of your net income. That is the maximum amount of your net wages that can be garnished at any one time by this type of creditor.
For example, assume you are sued first by Chase and then by Discover. And assume you net $4,000 per month. If Chase starts to garnish your wages at 25%, Chase would get $1,000 of your net pay per month until the judgment is satisfied. If Discover gets a judgment a month after Chase starts to garnish your wages, Discover must wait. Discover cannot simulaneously garnish anything because no more than 25% of your wages can be garnished under the facts you described. So if Chase garnishes 25% of your wages until it gets the money owed pursuant to the judgment, Discover would then commence its garnishment after Chase.
If Chase elected to garnish only 15%, Discover could garnish 10%. But it is really a race by creditors to get the first garnishment order because creditors typically take the maximum garnishment possible.