From optimization reasons more module isn't allowed on the same site. If you would like to insert a Register and a Login button into your site which can open the forms in popup, you should create a Custom HTML module where you insert the buttons.
<button class="loginBtn log-popup">Login</button>
<button class="loginBtn reg-popup">Register</button>
If you use elegant theme, you should enable the theme parameters / open popups at center parameter.
That's all, now check the result on your frontend.