How to properly layout an external antenna seating and on board antenna at the same time?
(1) is flawed, you would be better to go with (2).


  Isn't two "de facto" antennas better than one?


Depending on your power amplifier specifications, and the details of antenna match and line lengths, in the best case the total power emitted by both antennae will be less than from a single one due to the impedance mismatch, with nulls occurring at various frequencies and directions due to mutual interference, and in the worst case your power amplifier will be damaged. 

A setup designed to use two antennae will usually be better than one. However they key here is designed to, not just another antenna slapped in parallel. 


  ... is the 2nd pic overthinking?


No. The problem is that RF is not DC. (1) would be fine at DC. The problem is that even when you use the 0ohm jumper and socket to connect only one antenna, there is a stub of line hanging off the transmission line to the connected antenna. 

In the case of the socket, while the stub of extra track from the socket to the 0ohm resistor pad is fairly short, it is not zero. How significant it is will depend on the frequency of operation. A \$\frac{\lambda}{20}\$ stub will likely have a small effect, as it behaves like a small capacitance. It may still make your antenna deviate from specifications, and in a more frequency dependant way than expected. A \$\frac{\lambda}{4}\$ stub will be catastrophic, behaving like a short circuit at the socket.

In the case of the on board antenna, the residual length of the socket will behave like a stub of line.

Even if the effect of either stub is minimal, is the correct design of (2) so much more bother?