It's generally preferable for your character to be in one scenario at a time as usually the different scenarios will be happening on the same day at different locations. However, it is possible for you to leave scenario #1 and join scenario #2. Hopefully, this will also help prevent being post-locked.

Let's say Scenario #1 is a day out in the slums and Scenario #2 is focused on events happening within the Academy. Both of these events would be running alongside each other. You could start RP'ing in Scenario #1 and then have your character leave and return to the Academy. This would enable you to join #2 without creating a sort of anachronism.

Sometimes, Scenario #1 will finish before #2. This can lead to #3 taking place the next day. When this happens, only characters who aren't currently involved in any scenario should join #3 as otherwise, things may get quite confusing and it'll lead to the incomplete scenario being compromised.