If they are brothers then the rule is that the younger must not marry before the elder. Regarding marrying at the same time, nothing is mentioned anywhere AFAIK.
This is mentioned in all the Smritis. Here I'm using the Parashara Smriti (PS) as my reference.
An elder brother remaining unmarried while a younger marries, and that
same younger brother himself who marries before his elder brother, and
the woman who becomes the wife of such a younger brother, together
with the person who gives away the bride, and the priest who
officiates at the marriage, — all these are doomed to hell.
A double Krichchhra is the penance ordained for the said elder
brother, a single Krichchhra for the bride herself. He who gave away
the bride must perform a Krichchhra and an Atikrichchhra ; while the
priest must observe the Chandrayana rite.
PS 4- 25,26
However, in certain scenarios, a younger brother marrying before his elder brother is also allowed.
If the elder brother be a hunchback, a dwarf, a eunuch, an idiot, or
hoarse of voice, (probably the word
* gadgada ' here means ' a person of an indistinct utterance,' like the dumb), or blind from birth, or deaf, or dumb, there is no sin in
the younger's marrying first.
I don't know the answers for any of the other possibilities.