Both posts CF? If so I'll bet the larger post will need significant cleanup inside (ie sanding and filing) to slip over the broken post but I don't see why it is not doable in concept. Use epoxy, not a polyester based resin. Be aware the epoxies "flow" as part of the cure cycle. That flow is wonderful for filling gaps but that epoxy will find a way to split the scene it you give it an avenue. (Dependent on how thick the epoxy is, warmth both generated in the cure and added from the outside and setting time.) Taping the joint ends with masking tape will do a lot to keep the epoxy behaved and will lift off after cure.

Safety-wise - if you bring the joint down to say 5mm above the clamp I cannot see anything worse happening than the seat slipping down that 5mm if all goes to heck. (Assuming you are road riding and not all out extreme BMXing.)

Another thought - that I've had before but never saw a way around: I like my seatpost set so the nose of the seat is almost a cm to the left. Always saw the aero posts that slid into aero seat tubes as a big compromise for me. But! with your proposal, I could twist the outer post slightly askew and set the seat right where I want it!