In many instances, long distance lines have more than one platform track per line track. So dwell could happen in parallel. It's still an issue, but could be mitigated.

My guess is that outside of the peak, these kind of trains are not necessary. And the peak can apparently be accommodated without them. So there may be some times where they would be economically advantageous, but not always - meaning it could make sense to have some double deckers.

But for train services, it makes sense to have uniform fleets, both to reduce maintenance costs and increase dispatching flexibility. Trains should also have the same performance profiles and dwell times (as mentioned before). So any small peak time advantage is undone by the bad economics of having a non uniform fleet.