I think once the gotas KNOW they can escape and have the run of the place - they will always try and find a way. I also think if you only have just two they are much mor elikely to think they are an extension of the human family. Mine are in a group of now 6 (3 adult females and three kids) and get on fine outside. I took in a rescue goatling earlier in the year and she had been kept as pair, was a terrible jumper and otherwise a bit spoilt too. However, once she "tested" the electric top wire, got a stern "no" when she tried to jump the gate (I kept her on a lead and was present for the first few times outside with her new herd to get her used to the others) she never once tried to jump (but she was excellent at spotting open gates and led the other goatling astray...)
Mine have normal sheep netting plus one top strand electric - but mains. They have also never tried to jump any of the gates.
I am not sure tethering is the answer, but more electric ummmppph might be. Also maybe taking them out on a walk for a bit everyday, and otherwise restricting them to a small, but highly (6' +) fenced exercise yard. They don't need to run off their energy every day anyway....