Don't.

It can't be done.

If you can tolerate a delay in sampling your signals, you can buy what is called a "multiplexer" often abbreviated as "Mux". A Mux unit will take the two separate signals and switch them back and forth to your single input in a deterministic fashion. So you would need to accommodate that in your control device and have it only look at the proper signal when it comes in. this is done all the time on systems where response time is not critical.

http://en.wikipedia.org/wiki/Multiplexer

It depends what you are measuring,and what effect you require.If you`re looking for a digital signal at a pre-determined level of the measured value for example,it may be possible however,if you`re after a modulating output relative to 2 x modulating inputs,as the previous replies state,it cannot be done.

More info required with regard to the control philosophy and what`s being measured etc etc