Re: Is this TBH too small for backyard? Realistic to have just 1-2 hives?
"Is this too small?"
You can raise bees bees in that volume. Left to their own devices, bees look for cavities of about 40 liters, a bushel, or ten gallons in size. What you describe sounds to be in that range.
"Will it be a constant swarm risk?"
Short answer, yes. Some strains of honey bees tend to swarm more than other strains. Depending on the genetics of the bees you obtain, they will tend to swarm in a volume of that size. If you are concerned about bees swarming in a fairly urban area, you would be better off with a larger top bar hive. Having room to expand in the hive is easier on the beekeeper.
"Should I make a second one for spillover?"
If I were going to make a second one, I would go ahead and build it to the dimensions described by Duncan. But, having the same length top bar in multiple top bar hives makes things easier to manage. Probably not what you wanted to hear. I hope this works well for you.
"The brightest conceptions are insufficient without observation." Francis Huber