Transistor Voltage Switch - Circuit Help Please
What you want for solving the stated problem is a analog multiplexer.  The CD4051 you are already using is such a multiplexer.  The simplest solution would be to connect the two signal to two separate pins of that existing multiplexer.

If you really do need to select one of the signals onto a single pin of the CD4051, then you need a additional multiplexer.  In that case, it's not clear why the CD4051 was even mentioned at all, since it is irrelevant to your requirements.

Since you are already using a CD4051, just use another one to select between the two analog voltages.

However

This smells strongly of a X-Y problem.  The best system-level solution is likely at higher levels that what you are telling us about.  For example, unless this is something on loan from a museum, something is ultimately going to digitize the analog signals, then do whatever further processing digitally.  A possible solution might be to this new selecting in the digital domain.

Switching analog signals, especially ones this slow, went away with the 1980s for good reason.