Sorry, I guess aerials are only found in Europe.
Before cable & satellite, TV and Radio signals were broadcast from transmitters and received by metal objects which were of various shapes. These objects were called aerials.
For the best signal, the aerial was normally mounted on the roof of the house, i.e. high up.
The same word is still used to describe the metal rod that sticks up on a car (automobile) to pick up radio signals, that is sometimes replaced with a wire coathanger after someone forgets to put it down when going through a car wash.
The quality of the signal is referred to as the "reception". By having two aerials, the likelihood is that the reception would be better. "Reception" is also the word used in the UK to describe the meal/party held after a wedding. So we end up with a pun.
Unfortunately the length of this explanation is greater than the humour of the pun.
I think it's time I changed the joke.