The reason to wait a week or so before crossposting is to avoid having someone spend a lot of time trying to answer on one site, without realizing that it's already answered on the other. After a week, it's less likely that anyone is actively working on it.
Additionally, you must edit every copy of the question to include a link to every other copy.
Finally, make sure that your question is really suitable for MO: is it actually about research-level mathematics? There are lots of possible reasons why your question may not have got an answer on the other site. One is that it requires advanced or specialized mathematics knowledge, in which case MO could be appropriate. But it could also be that it is elementary but just difficult or tedious, or that you have not stated it clearly enough, or that it is based on serious misconceptions that are difficult to explain.
If you've already prematurely crossposted your question and are reading this later: it would be a good idea to immediately delete the other copies, if they haven't yet attracted much attention. If they have (in particular, you can't delete your question after it receives an answer), then at a minimum, make sure you have links as mentioned above.