In my case, I like to provision the OS boot drive at 150GB. But some systems only have 125GB SSDs. So I need to handle both. The way I handle it is during the Pro-OS, I have the following logic:
Map drive P: to preferred server scripts dir
IF ([Execute File action] diskpart.exe /s P:\diskpart_150.txt ) = 0
Do nothing (this is ok)
Else
(Execute File action2 diskpart.exe /s P:\diskpart_FULL.txt )
So the first IF condition is to execute a file (diskpart.exe) with parameter to run a diskpart script. The script is standard diskpart stuff and the important part is it tries to set the primary partition size to 153600MB (150GB). The diskpart script will return a 0 if everything works ok. If the physical disk size is less than 150GB, the diskpart script will exit non-zero and thus run the ELSE condition. That second diskpart_Full.txt script has a single difference which leaves out the 150GB partition size, thus using all of the disk.
In your case, assuming the computers you want to provision as such are the only units with more than one hard drive, you could put similar logic in place in the OS installation section:
Map drive P: to preferred server scripts dir
IF ([Execute File action] diskpart.exe /s P:\check_drive1.txt ) = 0
Run OS Installation with /d:1 parameter.
Else
Run OS Installation with /d:0 parameter.
The entire diskpart script for check_drive1.txt could literally just be "select disk 1". That should throw an error and drop you to the ELSE condition if there is only a single drive (drive 0). Just make sure you set the expected return value = 0 in the first IF condition and probably stop process the template if it fails.
Does that make sense?