Well... try looking at programs that offer both fellowships individually. They may be willing to work out a combined fellowship. Check the FRIEDA website for info on combined fellowships.
Have you tried the NMPRA website--the national medpeds residency association, at www.medpeds.org. They have a med-peds fellowship guide, and in the guide they keep a list of programs that have been known to accept m/pers doing a combined fellowship. I think with combined fellowships you have to do a lot of the arranging yourself. Maybe try to focus on programs that have med/peds residency so they're at least familiar with the med peds concept?