If the investors would appreciate the fact that they are the second person, then no one would have anything different to say about single founders, because any extra load would effectively be taken up by the resources that the investors give the true founder (adding someone to the team when there is too much load doesn't make them a founder just because it's early on, they can help in just the same way as if they were a normal employee).
Also the investor should (and is in a better position to) act in the same manner than a second founder does by encouraging and prompting the founder to deliver - it makes a huge difference when someone respectable with power is just saying "so have you got it done yet?". I think a second "founder" plays a large role in this respect, through just small actions.
