There is only one "coax" connected to the tuner. The coax runs from the radio to the tuner. Coax repairs have already been correctly described in this thread using two male connectors and a female barrel.
The cable that runs from the tuner to the back stay is not a coax (or at least it shouldn't be). It should be a high voltage single conductor wire. Splicing this type of wire can be tricky but it's possible to do. But high voltage cable is fairly cheap and if the existing one has insulation cracks and is broken then it probably got water in it some time and should be replaced. If you want to try splicing it you can do this with a regular Anchor crimp type butt splice. Heat seal after crimping and wrap with generous layers of rubber tape. Try to make sure that the splice does not come in close contact with any part of the boat but especially any metal bits.