Check with your Realtor. If the first buyer accepts your offer, then there's a signed agreement--a signed sales contract. And, yes, contracts are binding.
If there was a time period in your counter and that time period expires, then so does your counter.
If you're confident about the second offer, you can withdraw your counter to the first buyer and then accept the second offer.
You can tell the first buyers that you've received a higher offer. However, the first buyers could agree to your counter at the price lower than the second anyhow. So, strategically (and check with your Realtor) it might be better for you to withdraw your counter, inform the first buyers of a higher second offer, and see if the first buyers will go up.
Hope that helps.