If we attempt to count the trips made to the past in this movie, we run into a problem. We know that the departure of the stealth bomber from 1993 to 1943 is one, and although arguably it is undone by its own consequences we can still count it as the first trip. The problem is whether when Herdeg and German Mailer travel from the variant 1993 to 1943, they are making two separate trips, or only one. It is something of a technical question, but it is a significant one nonetheless.
We see Dave Herdeg leave from German Mailer's lab from the already activated time travel platform, and emerge in 1943. He is alone at that moment, and keeps himself hidden. Then in the future we see German Friedreich Mailer leave from the same platform a minute or two later, and emerge from the same portal in 1943, also a few minutes later. The logic of the situation suggests that there were two separate trips to the past. If so, though, they are rather complicated.
If Herdeg has left for the past first, then his arrival in the past changes all of history from the moment he arrived to the moment he left, and that must be resolved before anything else can happen in the future. He intends to destroy the stealth bomber, the consequence of which will be that the German history, including German Mailer, will be erased. (His departure from the future will also be erased, creating an infinity loop, but that is a separate issue for the moment.) In this resolved history, Germany did not have the bomber and did not win the war. German Mailer does not exist, and American Mailer is not building a time machine (at least, not intentionally), so even were it somehow possible for there to be another minute after Dave's departure, that entire world, as Jess anticipated, has been made never to have existed. German Mailer's time machine is not there, and he is not there to make a trip to the past.
Of course, it also means that there is no portal awaiting Dave for his return trip, but that is not our focus at the moment, either.
In order for German Mailer to follow Dave in a separate trip, Dave must have failed to destroy the bomber. He was seen, captured or killed, and never launched his firebomb. The bomber launches anyway, and the history Dave hoped to prevent is the history that happens, with one footnote about an American saboteur being thwarted in his efforts. Then German Mailer follows Dave into the past, and his presence distracts the guards sufficiently that Dave is able to get past them and destroy the bomber. Had it not been for Friedreich, Dave would have failed; he did fail. Because Friedreich followed him, Dave succeeded.
The alternative is a bit more complicated, but in analyzing Star Trek: First Contact we suggested that Enterprise and the Borg ship traveled to the past together in one temporal event. That means that the Enterprise never detected the changed world below them, because the Borg ship did not get there "first" and change everything for them to change back. Rather, using our present film as the example, the vortex created by the time machine is open at both ends for ten minutes, and at the future Dave enters followed by German Mailer, and at the past end Dave enters, and in the order that they entered the vortex expels them, Dave into the past followed by Mailer, and then Dave into the future. Thus we have a future in which Dave enters the vortex, and Friedriech follows him, and at the end of ten minutes the vortex closes and time bounces back to 1943, where Dave exits, followed by Mailer, all of history is changed by this one time travel event such that the bomber is destroyed and something like the original history restored. Dave then re-enters the vortex, passes himself going the opposite direction, and is expelled from the future end before it closes.
Except of course that it does not happen, because this vortex is never created in this new history, so we still have not resolved events.