engraving rules question - voices with merged heads

[Janek Warchoł]

Hi,

recently i was told that this notation

  { \mergeDifferentlyHeadedOn << b'2 \\ { b'8 a' g' f' } >> }

is not allowed in vocal music (i.e., the noteheads shouldn't be
merged). Is that true?

By the way, if there is a notehead shared by two voices (i.e. with two
stems, up and down), should there be two dots (one dot for each voice)
or one dot? Would one dot be a source of errors (it may look like it
applies only to one voice)?

thanks,
Janek