[The following assumes that Spouse A's LLC is not an S corp, otherwise Spouse A's 401(k) contributions are also determined by Spouse A's W-2, the same as Spouse B's. It also assumes that both spouses are under age 50 since you referred to a maximum employee contribution of $18,500.]
If you are asking in regard to contributions for 2017, the employee contribution limit for someone under age 50 in 2017 is $18,000, not $18,500.
The 20% employer contribution rate for Spouse A is the result of an employer contribution base rate of 25%. The employer contribution for Spouse B must therefore be 25% of W-2 compensation, except that total elective deferrals plus employer contribution cannot excess 100% of Spouse B's W-2 compensation, nor can the total for Spouse B exceed $54,000 for 2017 ($55,000 for 2018).
