Hello MABEs
"If I want to add a second Disk group to the host, can it consist of a 800 gb cache and two 3.84 TB disks?"
Yes this is supported but may have performance implications.
Assuming you have the same % used on both/all Disk-Groups, the larger ones will invariably be relatively busier as they are serving more data, as a result they will be under more pressure (especially considering you halved the cache:capacity ratio) - this means these will not operate as fast as the smaller disk-group and accordingly the speed of the smaller disk-groups may be affected (as these will end up 'waiting' for the larger, slower Disk-Group).
Please do consider that you are sizing your cache:capacity ratio appropriately for the design type, cluster configuration and workload requirements:
Hybrid:
https://blogs.vmware.com/virtualblocks/2017/01/18/designing-vsan-disk-groups-cache-ratio-revisited/
All-Flash:
http://www.yellow-bricks.com/2016/02/16/10-rule-vsan-caching-calculate-vm-basis-not-disk-capacity/
General guidelines on sizing:
https://storagehub.vmware.com/t/vmware-vsan/vmware-r-vsan-tm-design-and-sizing-guide/
A potential way of using larger drives but not impacting overall performance so much would be to use half as many double-sized capacity-tier drives per Disk-Group so as to retain the same cache:capacity ratio (and add more Disk-Groups if necessary) - note that simply adding larger cache devices (e.g. 2TB) won't really benefit here as vSAN is realistically only going to utilise 600GB cache per Disk-Group.
Bob