I thorny issue for sure. One suggestion is update the current investors that you will be leaving and you will create a succession plan that includes finding a new CTO (co-founder) to take on your role. For new investors, tell them clearly what that plan is, that you believe in the firm and you are willing to provide some of your shares to the new CTO. Accept that you will stay on until you and your co-founder can find this person. Be open to paying a headhunter. speed is important. Hope this helps.