Absolutely yes, you should work with active campaigns on both Kickstarter and Indiegogo to cross promote each other. Cross promotions were one of the best ways to expose our project to new audiences (we did two campaigns totalling $1.2 million across Kickstarter, Indiegogo, and BackerKit). For our most recent campaign, we raised an additional $1800 that we wouldn't have raised otherwise. We only worked with campaigns who were willing to promote our campaign as well, and agreed to do it for free. For the best results, we only partnered with campaigns that had a similar level of funding and quantity of backers. For example, if we had $100K in funding and 800 backers, we didn't work with anyone with less than $10K in funding and <50 backers. You want to be picky with the campaigns you promote to your backers list; you've earned the trust of your backers since they've paid money to your campaign, and you don't want to refer them to projects that are not fully funded, or look sketchy.