Can I split my SSD into two parts using Bootcamp?

Only if it is large enough and you use ONLY BootCamp to perform the changes.

DO NOT use any Windows-native applications that could change the partition map, or they WILL write over the Mac partition, leaving your computer unbootable.

It would be prudent to have a Mac backup before you proceed with any such changes

NB> Time machine does NOT back up Windows partitions, so you may need a separate Windows backup software.

BootCamp doesn’t care whether the boot drive is SSD or hard drive. Install the Mac OS on the new SSD and migrate your Mac software. Then run BootCamp to install Windows. Note - Migration assistant doesn’t back up or migrate BootCamp so you’ll either need to use a Windows program within BootCamp to do this or use WinClone

You can install Windows using Boot Camp Assistant or using a virtual machine like Parallels, Fusion, or VirtualBox.

Here are the Boot Camp instructions: Boot Camp - Official Apple Support

Yes

The SSD has to be your main boot drive. You did not say it was not. Read the instructions. I provided the link above.

Or are you asking if you can use the external drive to read/write from both Windows and Mac? If this is what you are asking you can format the drive ExFat. Both Windows and Mac can read/write to an ExFat formatted drive.