Communicating to 2 SPI Slaves with USART & SPI ports on Atmega16U2

I‘m writing code for an embedded chip that consists of an Atmega16U2 connected to two devices via the port B SPI pins and the Port D USART pins (which can be used for SPI comms as well).

I‘m a bit confused: is the SPCR register a shared resource that will be used for communications to both my Port B and Port D SPI slaves?

Also, i‘m a bit stuck on the nitty gritty of how to send data to a specific slave. Since I have two SS lines (DDD4 and DDB0) connected to the uC do I just bit shift one of them low and start calling my writeSPI() function? What would my readSPI() function look like?

C Code:

void SetupSPIHardware(void)
{
	/* Set MOSI and SCK output, all others input */
	DDRB = (1 << DDB2)|(1 << DDB1);
	DDRD = (1 << DDD3)|(1 << DDD5);
	/* Enable SPI, Master, set clock rate fck/16 */
	SPCR = (1 << SPE)|(1 << MSTR)|(1 << SPR0);
}

void writeSPI(char cData)
{
	/* Start transmission */
	SPDR = cData;
	/* Wait for transmission complete */
	while(!(SPSR & (1<<SPIF)))
	;
}