Is it possible to chain BMS modules?
Yes, you can. I do not see anything wrong with it except for the extra junk of communication cables and apparent higher cost. In fact, that is what happens in some big energy storage systems. It is easier to replace a single BMS instead of the single BMS for multiple smaller modules. It also helps with the integrity of the individual cell. Basically your risk factor is reduced hence is your cost over time.
For your case, If you just want cell balancing and protection, without reading the data, then go ahead with two 6s. Otherwise, it is cheaper and easier to get a 12S BMS and read the data with your computer.