I assume that one or both of you were covered under a Family Plan in your HDHP. If so, the IRS considers that both of you were covered under a Family Plan, even of the other spouse was on a Self-only HDHP.
The contribution limit for a Family Plan in 2017 is $6,750, with an extra $1,000 if the HSA owner is 55 or older.
Note that this applies to the family. That is, the sum of the HSA contributions to both taxpayers' HSAs cannot be more than $6,750 (or $7,750 if 55+).
Your $9,400 aggregated contributions exceed the limit.
The only wrinkle is that if you were both covered by separate Self-only HDHP plans, then each of you would be able to contribute $3,400, for a total of $6,800 in the aggregate.
But since you contributed $6,000, I am assuming that someone has a Family plan HDHP.
