Question: Ok, I have one uart interface (TXD GPIO 14, RXD GPIO 15). I want at least one more uart interface.

Possible solutions:

* Bit banging:
Use two unrelated spare GPIOs. I understand that timing is a problem on a standard linux. Would it be reliable with very low baudrate?
* Switching:
RPI decides when to talk to which device. Using e.g. [CD4066BC](http://www.fairchildsemi.com/ds/CD/CD4066BC.pdf).
* spi to 2 x uart bridge:
Didn't find a suitable part (availability, price, dil package)
* usb to uart:
Expensive

Are there other options? I'm inclined to switching, if it can be done. What would you advise?

Answer: 
I had the same problem. I need connect to 2-4 GSM modules and I found hardware solution: <http://www.instructables.com/id/SPI-to-4-x-UART-Bridge-MULTIUART/>

This solution are base on PIC24FJ64GA306. You can replace PIC with Atmel mcu, but You must create new PCB :)
Another solution is to move up to a PI4 apparently you can enable multiple uart pins on that one (did it myself)
You can google the info up, it is mostly a config setting to enable the additional uart pins.

<https://www.raspberrypi.org/forums/viewtopic.php?t=244827>

This explains it.
Apparently the new gpio pins in the pi4 basically allow to add 3 additional uart ports via the config.txt, for further details on how to set it up, check the link.