"While they were travelling abroad, they were making a lot of friends" or "while they were travelling abroad, they made a lot of friends"
Repeating the continuous form in contexts like While [verb1-ing], subject is [verb2-ing] is relatively unusual. And precisely because it's a less common choice, native speakers will go to some lengths to infer a specific meaning to that choice (if they know it was a competent speaker's deliberate choice of phrasing).
The assumed implication will normally be based on the idea that repeating the continuous form expresses a stronger link between verb1 and verb2 than mere "simultaneous activity" as already conveyed by while (or similar).
Thus in the exact cited example, I'd probably assume the speaker wanted to imply that the fact that they were traveling abroad somehow made it easier (or otherwise more likely) for them to make new friends. But it's not necessarily a good example. For more straightforward alternatives, consider...
1: While studying for my degree, I wasn't earning much money
2: While working as an unpaid lifeguard, I'm getting a good suntan
...where we might reasonably infer because linking "downside" low wages to studying in #1, and although linking "upside" getting a tan to doing a job without pay in #2. Note that in other contexts, different relationships between the two activities might make more sense.