The price is determined by supply & demand.Prices fall...

The price is determined by supply & demand.
Prices fall dramatically when there is excess supply.

Private enterprise decides how much of something it will produce to make the maximum return for it's shareholders, as is the fiduciary duty of it's directors.
If renewables decide not to flood the market with excess power so as to lose money, that's their right & their obligation.

You want a parallel bidding system that would achieve nothing.
Coal would still be more expensive & still wouldn't be used until renewables supply is too low due to constraints on production (ie lack of light or wind).