I think, you just set a parameters for aggregate, in a time of creation, and choose a RAID group size = 8 disks, so, after installing a new disks, you've got 2 disk added into first RAID group, up to 8 disks in that RAID, then system start a new RAID.
I don't know how to merge that groups into one, I believe only - backup/reconfiguration with RAID group size = 16 (default for SAS)/restore - will help.
If you decide to keep the two RAID groups rather than rebuild the aggregate, you can always add to them later with a SAS shelf. You will have to add the new disks from the command line and specify the RAID group. By default, ONTAP only adds disks to the last RAID group created until it becomes full and then create another RAID group rather than go back and fill up rg0.
I believe you can’t do this, In your case there are two raid groups rg0 ,which is full with (7Data+1Parity) and the rg1 with 2 Data + 1 Parity. The reason behind the second raid group got created is the raid group size which you have created during the aggregate create was default 8 for RAID4. Its 16 for RAID-DP for SAS disks. I would recommend you to fill your second RAID group rg1 during expansion.
Migrate to RAID-DP !
If you want to migrate to Raid-DP you need to have 2 more parity disks in your case one will go for rg0 and the other one for rg1. Mike has already given you answers to this.