I'm not entirely sure how you want to filter (I'm assuming that both columns need to meet both criteria but I would suggest creating a helper column with something like:
=AND(LEN(A1)<=16,ISNUMBER(SUM(IF(FIND(MID(A1,ROW(A$1:A$16),1),"0123456789abcdefghijklmnopqrstuvwxyz",1),1,0))),LEN(B1)<=16,ISNUMBER(SUM(IF(FIND(MID(B1,ROW(A$1:A$16),1),"0123456789abcdefghijklmnopqrstuvwxyz",1),1,0))))
as an array formula (confirm with ctrl+shift+enter) and then filtering on that column.