What is the correct way to attach two grounding sources to my electrical panel?
If in doubt, don't be bashful about adding an accessory grounding bar or two. This attaches direct to the chassis of the panel.  

This can be a wiring convenience, because you can terminate each of the cable's grounds at the grounding bar, rather than taking it all the way to the breaker with the neutral. 

Separating the grounding bar from the neutral bar also aids making this panel a subpanel in the future; 24-space is worryingly small for a whole house, so that may be an option if/when you outgrow it.   The reason you can't just turn it into a 48-space using "double-stuff" breakers is that as of NEC 2017, most circuits must be AFCI or GFCI, and as of NEC 2020, virtually every circuit must be.  AFCI breakers are not possible in double-stuff, so every 120V circuit will need a full space, and every 240V circuit will need 2.   You might be grandfathered in that respect today, but if you do any renovation you'll quickly find yourself out of spaces.  Again. 

I am also a big fan of 100% separate neutral and grounding bars, with a clear neutral-ground bond that is a wire.  That lets me clamp an ammeter around it to look for ground-fault leakage.  



To the extent able, you should leave hots and neutrals long enough to reach every breaker space thay can. That will allow you to a) upgrade the circuit to AFCI or GFCI, since they need neutral at the breaker; and b) move the breakers around as needed to acommodate many things which need to be in a particular place - whole house surge suppressors, gen interlocks, solar power breakers etc.