I have switched 4-20mA signals using a standard electro-mechanical relay. I connected the normal signal to a NC double-throw contacts, and the 2nd signal to the NO contact. It was fast, but not instantaneous, and I did not see any problems with bounce.
I think I used a timer and told the PLC to keep using the last value of the old signal for 2 seconds, and then switched over to the new signal 2 seconds after the relay trigger. In effect, the PLC timer becomes your semiconductor relay.
I may have mis-understood what MrQ needs. I thought it was a case of Either-Or of 2 signals feeding to 1 PLC signal input (switching between 2 signals), not both signals multiplexed in serially one after the other, one-per-time-period.
Depending on the transmitter you could get an upset if they are loop powered and the loop breaks. Some take a bit of time to re-start.
How about using a voltage input PLC, have both transmitters feed a resistor (250 or 500 ohm)each then switch the voltage signal from across the resistors. The loops stay active and stable. I'd use gold plated relays to cut the noise.
I did it years ago, when AI was expensive and I was out of channels and expansion. In today's world I'm not sure it pays. Having said that, you will need to allow time for the transmitters and signal to stabilize after switching, and you will need to make sure your relays break before make.
Gold plating is excellent for corrosion resistance but can wear through in time. Silver is more conductive but can be prone to oxidation. Mercury wetted reed relays can provide low current / voltage capabilities even though mercury has less conductivity. Cross bar contact relays can also provide low current / voltage capabilities.
I told the customer that I will not go to the former Soviet republic where the machine will be placed if it runs in to problems due to trying to save one AI and then it was no problem..........
Consider using bifurcated relays for an application like this. It will help ensure the accuracy of your 4-20 over time. We have thousands of these multiplexed I/O points installed and the bifurcated relays really do make a difference in reliability - we learned this the hard way...