No, a Martin Yale 1217a autofolder will not trifold a previously bifolded piece because it has only a single feed wheel in the center for paper pickup from the top while a lower fixed rubber non-rotating "wheel" via friction performs separation. Thus unless you hand feed the already-bifolded pieces, the automatic feed will make the already folded pieces go askew. It can double fold smaller pieces ok (e.g. take a small sheet into 4 for announcement cards) as both side rails of the input rail slide from side by side to move the feed wheel to one side right over the fold, but this only works for pieces half the size or smaller as the total input width is ~11". Hope this helps.