Buying more tanks becomes complex because you then need to construct a "cascade" system to equalize their pressures and flow rates. You're much better off just getting a larger tank.
A "hypoxic room" is very difficult to construct outside of a laboratory or industrial setting. Creating a sealed space of any size, and then providing adequate gas to first flush out the air and then maintain a CO2-free atmosphere, is beyond most of our capabilities.
The one thing you might explore if you want to pursue large-space inert gas hypoxia --and you would be on the cutting edge for this-- would be the use of liquid N2. That is a much more concentrated form of N2 than compressed gas, and you might be able to use it to purge a large space of air within a reasonable amount of time. You'd need to balance the hypoxia against hypothermia, however.
Liquid N2 is available at industrial gas suppliers like AirGas --veterinarians use it for cryo storage of semen; some women attempting to get pregnant via donated sperm use a dewar flask with liquid N2 to keep their doses frozen-- and I don't think you need a license to get it.
I can make guesses at how to go about applying liquid N2 to ctb, but I have no certain knowledge.
