I should add the caveat that I usually go on the well-traveled short routes where they can add buses at peak periods. Sometimes rather than having a bus number (e.g. 48-15) it just reads "spare." That takes some of the pressure off.

I always put my carry-on bag next to me...it's a zippered duffel-type roughly the size the airlines allow and more malleable. The bins aren't usually wide enough to hold it and it's too fragile (and often too "valuable' in that some stuff could be stolen) to put in the hold. I've also had a time or two where the bus hit a rainstorm and water got into the hold.

Never any problems at the ticket counter...if you say 2 they assume it'll be together in the same row or they'll bump you down to the next bus. It sometimes confuses the conductor (who thinks a passenger is missing), though I put both tickets on the bag so she'll see.

It's also much better to do with the non-stop routes...the orange ones that pick up people along the way don't work well with that.

Have also seen some VIP buses on the Isaan intercity routes where it isn't necessary (i.e. you get the wide seat and 3 to a row). I once bought 2 when I didn't have to on one of those and ended up with the front 2 on the top level of a VIP double-decker bus (Khon Kaen to Khorat, IIRC)