For Serendipity I made a three part mast from aluminium tube of different diameters and I presume the gluing principles are the same. Applying the glue and sliding the two tubes together is easy. I found in the past, with aluminium, that a much stronger glue join results if you sand the surfaces and prime with liquid epoxy - then, before it is cured, paint on the epoxy glue and slide the tubes together (making sure the mast is lying straight.) Where there are two different external diameters (as in your top section) the join should be faired with filler and glass tape, to make a smooth transition between the two diameters. I put a bandage of tape over the join itself, also.
In my opinion (others may not agree) the important role played by the glue join is just to hold the two parts in place, resist twisting and resist the top section being forced down into the lower section. As for bending forces, I do not believe the glue join needs to contribute much, provided the inner tube is sufficiently "buried" in the outer tube - the same principle as applies to a tabernacle. This means, in theory, about 30cm each way (your 60 cm joiner would do it) - but intuitively I would rather double that. With lots of gluing surface and a material which should take kindly to to epoxy, there should be no problem with the join.
I would leave it to the experts to advise you about whether 50mm carbon fibre tube is strong enough for your mast. I wish I could get some of that tube.
Heaving down with the halyard. It puts a bend in the mast, but the join is plenty strong in bending.
