NIH added a new category a while back: multi-PI proposals. NIH is making the distinction between roughly co-equals (ie co-PI) and folks who are senior and contribute to the work but are not taking a lead role (called co-I's). (My understanding is that co-PI counts towards tenure/promotion and co-I does not. YMMV).
The overview from NIH:
The multi-PD/PI option presents an important opportunity for investigators seeking support for projects or activities that require a team science approach. This option is targeted specifically to those projects that do not fit the single-PD/PI model, and therefore is intended to supplement and not replace the traditional single PD/PI model. The overarching goal is to maximize the potential of team science efforts in order to be responsive to the challenges and opportunities of the 21st century.
There is a lot of information here, with many sublinks.
For review criteria NIH says: "Standard NIH review criteria accommodate both single PD/PI and multiple PD/PI applications", there are a few additional requirements that feed into the review. One of these is the Project Leadership Plans for Multiple PI Grant Applications. And, of course, there is lots of information and even some good examples.
Which brings us to a comment about writing length from girlparts was:
I never used to have this problem (being too long) -throughout school, early years of grant writing - page limits were never an issue. But lately, with the rise of the multi-PI proposal, it has become a problem for me. The scope of the science is bigger with two-three labs involved. If they are also cross-disciplinary proposals, they seem to require more background information. You might have one reviewer in your field, who wants all the nitty gritty experimental details, and then someone in the other PI's field who has no idea what you are talking about, or its significance without an overview of your whole field.
I wish I knew how to make it work. I'm thinking that I should take a similar approach to that of cutting down from 25 to 12 pages - mostly less technical detail?I'll try to step back a few levels and state the general goal of the experiment, and not so much of the how, with references to our previous studies. It's a multi-submission approach: if the reviewers come back with specific technical questions, they are the easiest kind to answer. Fingers crossed.
I think she hits it on the head. You cannot stint on the significance and innovation. And part of the significance is why this is a multi-PI proposal. I agree that reducing the technical detail to include more on the justification is probably the better road to take.
Yet, one important additional consideration is that if the project is too big, even with/despite multiple PIs, you can still be dinged for "over-ambitious" . It may be time to think about other mechanisms, such as program project grants (P-grants) or even just more than one R01. If you can't include the necessary information in the 12 pages, then either 1) you are stuffing in too much detail (and indeed, page lengths are your friend) or 2) the project is too big and you are stuffing in too much big picture.
One way to tell that your multi-PI proposal is Too Damn Big is if you can't figure out how to reduce it from 5 aims to 3. There is a lot of information/thought/chatter on how many Aims is the Right Number of Aims. Make sure to read the comments, there is some very subtle NIH zen in there about substance versus organization of substance.