You could create a separate object which will have a reference to the trip and the passenger. You can then use that string parameter to retrieve the object that has the combination of the passenger and the trip. The other option is to have one string and use the substring method to split the strings into two variables. E.G.
substring($inputString,0, find($inputString,'&')) - first part of string. substring($inputString,find($inputString,'&')) - Second parameter of string.
Hi there, did you see this post? It describes how you could use your own strings to retrieve the right object in your microflow.