How to split Time Machine backup between internal and external drives on new iMac?

You can not split up the migration from a Time Machine backup onto two different drives.

Install macOS on the external 8T SSD, startup from the 8T SSD and then migrate from Time Machine to the 8T SSD.

How to use an external storage device as a Mac startup disk

Then you can choose to run from the 8T external SSD or run from the internal 1T drive and access the data on the external 8T SSD.

You cannot direct a migration of Mac data to multiple drives nor to an external drive.

You may have to manually transfer your data from and to the locations you wish.

What is the physical setup of the old iMac? Is the 6 TB drive the internal boot drive?

If the 6 TB drive is an external drive, it would be easy to connect that and the 8 TB drive to your new iMac and directly transfer your stuff.

Otherwise, your options are to connect the two computers via network to transfer files, or by Thunderbolt and target disk mode. The latter would require about $100 worth of cable and adapters to make happen.

Or you could buy another external drive to connect to the old iMac, manually copy its files and then bring them over to the new iMac.

I, myself, would pull the 6 TB drive from the old iMac and use that directly with the new iMac.