I know a lot of history, and so I know a lot about what was happening at different parts of the world at different times.
And I still find myself surprised from time to time to realize that two different events in different countries happened at the same time.
So you probably need charts with time lines to show how events in the experiences of one character relate to the events happening to another character.
Here is a link to some examples of timeline charts for history.
So you should make a similar chart with either a vertical or a horizontal flow of time.
The vertical or horizontal time scale should have equally spaced marks for each minute, hour, day, week, month, year, decade, century, or whatever, that passes in your story.
Some novels might happen in mere hours, in less fictional time than it takes to read them in real time, with many characters having experiences in different places at the same time over a short fictional time span.
Other novels could be epics in which long lived characters or gods decide the fates of entire civilizations over a span of many centuries or millennia.
And possibly different parts of your novel will have events happening at much different rates than others, and you might need several timelines with different time scales for your novel.
And in your timeline you would write the experieces of the characters at the times when they happen, or maybe with arrows pointing from the descriptions to the exact moment where they happen.
And if you are like many or most people you might underestmate the size you need for your timeline and cramm it full of cramped illegible notes, and have to start over again on a larger scale so it will be readable and have all the necessary information.
J.R.R. Tolkien was a very carefull writer when writing The Lord of the Rings. The Return of the King has a set of appendixes giving data about Middle earth. Appendix B The Tale of Years, gives chronologies of the Second Age and the Third Age.
It has entries like:
1980 The Witch-King comes to Mordor and there gathers the Nazgul. A Balrog appears in Moria, and slays Durin VI.
1981 Nain I slain. The dwarves flee from Moria. Many of the Silvan Elves of Lorien flee south. Amroth and Nimrodel are lost.
And it has a section called The Great Years, that cover the events of the main action of The Lord of the Rings, happening in the years Third Age 3018 and 3019.
For example, in TA 3019 March:
1 Frodo begins the passage of the Dead Marshes at dawn. Entmoot continues. Aragorn meets Gandalf the White. They set out for Edoras. Faramir leaves Minas Tirith on an errand to Itilien.
So that entry says what four different groups of characters did on March 1, 3019.
I don't know how much Tolkien wrote out his chronology ahead of time and how much he made it up as he went, but he revised the novel several times and one of his goals was to make certain the passage of time added up, so I'm sure he made several versions of the chronology while he was rewriting.
There are several points where the narrator says that while one of the protagonists was doing something, his friends far away were doing something else, unknown to each other.
I have noticed only one chronological descrepancy between The Tale of Years and events in the novel, and as far as I know other readers never noticed it.
So J.R.R. Tolkien should be a good role model for someone writing a novel trying to keep track of simultaneous separated events.
And other answers suggest using various types of computer programs.