Most applicants to med-peds will not have done both acting internships/sub-internships in both internal medicine and pediatrics. Since most schools require AI/subIs in medicine, that's the one most commonly done (and an elective in pediatrics, although some 4th year peds elective are automatically counted as a peds AI/subI)
Many applicants get interviews and match into great med-peds programs without having done both AIs/SubIs in medicine and pediatrics
good luck