Should I put 2 layers of felt on my garden shed?

You should. But the underneath one should probably be an underlay type without chippings embedded in it. The usual method is to tack on the first layer and then glue the second one on top. If you've glued on the first layer you could get away with just that, but I would definitely use two. Note: you can get very soft thin felt, which seems to often be used on sheds. This tears easily and is quite useless - not sure what it is really intended for. The type I have used before is a few millimetres thick and includes some type of small chippings in the surface. Having said all that, glueing several layers of anything together with the appropriate glue should be fairly watertight ! Simon.

This is used for flat rooves, and is rolled out into molten bitumen, then molten bitumen on top and another sheet rolled out in the opposite direction, finally, molten bitumen and green mineral felt rolled into that as a finish, or the smooth stuff, pained with aluminium paint.

see above, green mineral felt

A suggestion I got from the group is working well four years later no leaks ... and that is to put down a sheet of thick plastic and then put the felt on top. The plastic is properly waterproof and the felf stops sunlight and wear degrading the plastic

Anna