Do we need to have 1 million baht in our bank account when applying for a DTV as a couple?

In principle, yes. The 500k baht equivalent is in generic requirements, meaning that it applies to you both. However, some embassies are a little more lenient and do not require the money for dependents. It is worth checking.

And even if the money is required for you both, you could still get around it by using the same 500k twice. That then depends on the seasoning requirement, which also differs per embassy. Some do not require any seasoning, so you could just send the money to your spouse to support his/her application. You can't apply simultaneously anyways if your spouse applies as your dependent; it must then be done sequentially.