Yes the second bus waited until all of us that had been in the first bus had passed border control. Since they were taking in batches of 20-ish people every time, he only had to wait for a few batches of people for everyone from our bus to pass through. I assume that the driver of the second bus knew of the length of the queue and probably didn't wait for the entire 7 hours, but I didn't ask.
Also you're right to take an earlier bus! Since mine arrived at the border around 14h00, we had finished the crossing at around 21h30, we arrived in St Petersburg not too far from midnight, so had I not found someone on the bus who agreed to exchange roubles for some of my euros, I wouldn't have been able to exchange money in St Petersburg (the banks and whatnot were already closed)!
I travelled with Lux Express, simply because it's the company I had heard of when I made my impulsive decision to go to Russia. We had to go to another bus that was waiting for us on the other side of the border (from what I understood it's because the border was under repairs, so the bus couldn't physically cross it; otherwise we would cross the border while on the bus, leaving just for the passport and baggage checks: this is what I did on the Kaliningrad-Gdansk bus that I took on the way back). The contrast between the two buses was actually very large: wifi, chargers, screens and coffee machine on the first one; not even chargers on the second one; so if your buses are similar, don't repeat my mistake and do charge your phone in the first bus just in case!
The queue was so long, it lasted 7 hours! And it rained, so even if it was the summer, it was pretty cold! Thankfully some Russians who were travelling in the same buses decided to give vodka to everyone to keep us warm (I don't drink under normal circumstances, but I made an exception there). Perhaps me being tipsy contributed to why I didn't declare my cash properly lol. At least, once we reached the border itself (after the long queue), the crossing was rather quick.
However, my bus dropped me at the border in the early afternoon, so I imagine that in your case there will be fewer people since you'll arrive in the early morning. I was told before the crossing that it would last 4 hours, perhaps this is the time that you should expect to wait?
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