What is the fair % to give to a new CTO/web developer entering in the company after 2 years the company has been incorporated (no revenue) ?

You might be somewhat in trouble. You'll want to consult a lawyer, not to be adversarial, but rather to ensure you do things that are legal and are best from a tax-consequence perspective. You can't back-date option grants or stock grants, for example. There are recent scandals reinforcing that statement. And if you don't back-date, your stock (or options) is worth something now, and might even have to get a 409A to ensure it's valued properly. (That's a 3rd-party stock valuation report, typically costs $10k-$30k, which companies with ESOPs are required by law to do annually, but I wouldn't be surprised if this company hasn't yet.) There's all sort of other boogers which you couldn't even know about. For example, if an outside party made an informal written offer to buy the company, even if it went nowhere after that, that will change the 409A valuation. So this is why, in short, I'm giving you the unsatisfactory answer that you'll have to use a lawyer to make sure everything is done properly. This is, sadly, a object lesson in getting paperwork done properly from the start. I'm sorry to be the bearer of bad news!

Type of shares are generally split into two: common/ordinary (depending on the jurisdiction) and preferred. The former are granted to founders and employees (most commonly as options with a right to purchase common/ordinary shares). The latter are generally issued to investors and include special rights such as liquidation preference, anti-dilution protection, etc. Preferred shares reflect the fact that the holder (investor) has paid a premium in consideration for issuance of such shares. Don't forget to consult with a lawyer.

This issue could be something the founders are unable to control. Now it has become a trend that founders (and early employees) can liquidate some of their shares on a Series A or Series B round. Investors found this a necessary step, so the founding team don't think of selling out the company too soon just because they didn't have any money. This article would give you some guidance on the matter (and also a few situations you have to beware of.)

If they are offering equity as your payment then I'm surprised that they didn't come with a number. Or maybe they will. It's hard to give a firm answer because it all depends on how much money the two founders are investing on their own. If you say you'll invest equal portions then you can ask for 1/3 of the company. If you're just investing time, how much time, compared to how much of their time plus equity. Depending on those answers, the amount of equity you're worth can drop pretty low.

I've been a co-founder of three ventures and have had to negotiate ownership stakes with my partners for each one. I've also conducted intellectual property diligence, as a consultant to IP/M&A attorneys, for multiple multi-million dollar software M&A deals. As Stoney said, it's a bit hard to come up with a firm number, given the number of unknown variables involved here. Also, it almost sounds like you and your product are being acquired by this larger company, while essentially selling or licensing a product you've built to this company in the process. If that's the case, you may want to speak with an attorney who's experienced in software licensing agreements and M&A. They may be able to steer you in the right direction of a valuation, as well as make sure you're very clear in what you're giving away, from an intellectual property rights perspective. Another thought that comes to mind is that if the company is providing you with a lot of support, including a salary, be prepared for them to possibly low-ball you on the ownership stake. Once you decide on an ownership stake to suggest, I wouldn't be afraid to start at the high end of your estimate, taking into account the fact that they may try and negotiate the share down quite a bit, given the financial contributions they'll be making. In my experience, investors putting money on the table typically and unfortunately don't value sweat equity too highly or fairly, when compared to cold, hard cash. Always happy to discuss further on a call and good luck!