Why two passenger airplanes travel side by side?
The top one is Baltic 634, the bottom one BT684.

As you can see they had 2000ft seperation. That's 600m for us europeans not accustomed to aviation.

Furthermore, at 17:40:19 BT634 reported in at 52.409599,16.981899. 17:40:25 BT684 reported in at 52.423222,17.019314. While those are 6 seconds apart, the planes were north-eastern course at the time, so I guesstimate that they had 1-2km horizontal seperation as well.

As to why - they're going the same place. The separation is big enough that there's almost certainly no benefit to it.