Where are all the multiplexers?
CLK &amp; SDA are I2C signals, not SPI.  SPI uses CLK, MISO, MOSI,and chip select (slave select) to let the slave communicate.  CLK, MOSI, and CS must be isolated to not drive at the same time. MISO comes from the slave and connect back to both masters. You can use two 74HC125, which is a 4-gate buffer, with each gate having an output enable.  Enable one chip or the other to let the three signals thru to talk to the slave. Cost: 48 cents each.  DIP and SMD packages, as low as 37 cents a chip. https://www.digikey.com/products/en?keywords=74hc125