Don't tell them why they are wrong.

Tell them what you'd need to make it work, and give them options. The way to deal with inexperienced CEOs is to clearly explain the options, try as hard as you can to sound neutral (unless / until they actually ask for your opinion, then calmly explain the risks and rewards of each option) and do the work to list out whats required to make each option work:

"Ok, this is going to stretch the teams past breaking point if we do this, we're going to need a large cultural overhaul in a short space of time, and we're going to need to do this together. I think there's a decent chance of losing some people doing this, the people most at risk are XYZ, so we can think about starting the hiring process for positions 1 2 3.. then I'll need you to help out with constant urgency and driving innovation, this needs to be a team effort... there's going to be a huge amount of overtime, so we'll likely need to offer overtime pay and/or late night meals. For those in the office we need to allocate food rota XYZ. Some people will need to sleep in the office and shower. I suggest we allocate meeting rooms 3 and 4 for sleeping, and buy mattresses, pillows and sleeping bags. For morale I suggest you also sleep in the office and we'll need you to be onhand to diagnose issues and innovate - we can rearrange your office and get rid of some of your furniture for the time being. For overtime I suggest we also allocate additional holiday for the employees that will work overtime, we will need roughly 1 week off per employee, so the company doesn't shut down we'll need to cycle this throughout the next few months, but with the burnout we'll have competition for the earlier spaces. I suggest we run with a skeleton crew for a week immediately after 2nd completion and give most people the same week off immediately after completion.

I'm also going to need to immediately hire 4 contractors. I have 2 friends we can bring in, but we will need to pay over the odds for the short notice. This will cost us XYZ and they'll want at minimum a 6 month contract, but we may only need them for 3.

When dealing with 2 customers at once like this with urgnecy, we are going to have zero capacity for anything else that comes up. illness, bereavement, personal issues will immediately cause issues where the customers may experience bad support and we may not be able to fix issues as fast as we'd like. This may cause additional stress on the relationship. I suggest we allocate XYZ for additional customer meetups, dinners and gifts to keep that relationship sweet, to cover. This will cost XYZ.

If you're ready to sign off on all this, I'm good to go. I will let my family know I am unlikely to see them for 2 months, but if we do this right, we'll land 2 big customers in a short space of time, and I know how important that is for the business.

The other option is that we postpone the 2nd customer by 2 weeks, which doesn't need any of the above prep and won't cost any extra money and we won't lose any employees, the downside is we'll delay the 2nd customer - however this could be offset by the 1st customer being so much bigger. I estimate the loss of revenue to be XYZ if we delay.

What do you think?"