Assembling two bags thus in the field is a bit of a challenge, duct tape may be about the only practical option. However, if you are building kit at home and want to make something in advance, try using silicone RTV as the fusing media. It is flexible enough not to break and running a bead under compression would eliminate bulkiness. It may not fold up quite as compactly as the bags did beforehand, but it should be functional.