This method of dividing the total trip cost by the number of travelers would only work if the cost for each traveler is exactly the same (airfare, hotel expense, cruise expense...) If however the expenses vary, then the above method will not work, and it will result in some travelers being over-insured while others are under-insured and this will impact the reimbursement in the event of a claim.
Here is an example: John and Betsy buy airline tickets for themselves at $1000 each and a ticket for their son at $700. Therefore the total ticketing cost is $2,700. If however they divide the total by 3, and insure each of them for $900, John and Betsy will not receive their full reimbursement of $1000 in the event of a claim. This in spite of the fact that John and Betsy show $1000 as their expenses, they will be reimbursed for $900 only as this is the amount they are insured for. Likewise even though their son is insured for $900, his claim will only be $700 as this is the expenses shown in his receipt. In other words here the parents were under-insured while the son was over-insured.