The time reference comes from each individual log entry. In order to relate the results for host A and B together, you would need to be performing some kind of joined search where you take results from host A as your principal data source, and then cross-relate to matching results from host B. How you do that depends on what the data is, and what the fixed relationship is between the two machines.
Give more details. Right now it's hard to understand your exact scenario. All events will have a _time value set - what do you mean by just using _time from a certain host? How will that be used for other events?