Can I install two new BIOS versions simultaneously?

No. Doing them all at once will break the motherboard CMOS. You would download/install each one manually, one at a time, allowing the PC to restart by itself after each one. I would not use the SA (SupportAssist).

But, before you install any BIOS update, consider this...
* If the PC is out of warranty, only install a new BIOS to correct a specific issue you are currently seeing. You should never update the BIOS just because a new one is present
* If the PC is out of warranty, and the BIOS update fails and breaks the motherboard CMOS, Dell will not replace it. So are they worth the risk? Your call

Desktop Instructions =
* Download/Save the BIOS file to the Windows desktop
* Disconnect all external USB drives from the desktop. Leaving only the following devices connected to the rear of the desktop =
monitor
wired USB mouse
wired USB keyboard
* Closed all unnecessary software applications (Steam, Anti-virus, etc. )
* Find the saved BIOS file on the Windows desktop
* Right click the file and choose, "Run as administrator". Note, you do not need to "extract" the file. Just run it
* Follow the prompted instructions. The desktop will reboot on its own. Do NOT force it to reboot or power off

XPS_8700_BIOS_A08.EXE
https://www.dell.com/support/drivers/driverdetails?driverid=45DHF
7/24/14

XPS_8700_BIOS_A10.EXE
https://www.dell.com/support/drivers/driverdetails?driverid=WVHKG
12/15/14

XPS_8700_BIOS_A11.EXE
https://www.dell.com/support/drivers/driverdetails?driverid=TDDNW
7/31/15

XPS_8700_BIOS_A12.EXE
https://www.dell.com/support/drivers/driverdetails?driverid=4DJMV
2/23/18

XPS_8700_BIOS_A13.EXE
https://www.dell.com/support/drivers/driverdetails?driverid=3081M
7/2/18