I'm contemplating design where there would be 2 at386 devices that would be connected over I2C (or other). (One device provides optional functionality).
I would like both devices to have access to an SD card - the access would be coordinated between the devices.
I would also like to know the possibility of having multiple masters for the I2C - same as above but using serial Flash Memory. In this configuration, i would assume the two masters would need to coordinate over software serial or spi.
An SD card uses SPI not I2C. Therefore this advice concerns SPI not I2C.
Yes you could do this by making the device not driving the card have its pins switched to be inputs. However, if both devices were to be outputs at the same time you could damage one or both devices. Therefore I would recommend using a data select device like a 74LS154 to switch access of the SD card between the two devices.