How do you obtain both helix and dome fossils?

You actually can only get one or the other. You cannot get both within the game. The choice is up to you.

Hope this helps!

In firered, you can get both fossils by using the same code that is used to get all three Starter Pokemon. You have to click yes you want a specific fossil, then as it is saying you obtained if, click your cheats, activate the code, then just step over and click on the other fossil, turn the cheat off, click yes, then after you will have both fossils. If you didn't know, that is also the same way you do it when getting all 3 Starter Pokemon. The code I used was 83000F9C 0000. They are all zeros no letter o, btw.

There is another way to get all three fossil pokemon. Just use the PkHex to edit the flag.
This is the event flag that you need to edit:
47: dome fossil Overworld -Unset to make it reappear
48 - helix fossil Overworld - Unset to make it reappear
626 - Choosing Dome fossil - Unset if you choose helix fossil, Set to resurrect Dome Fossil at Cinnabar City
627 - Choosing Helix Fossil - Unset if you choose dome fossil, Set to resurrect Helix Fossil at Cinnabar City

To make it work ya'll need to download pkHex and edit using event flag at the research tab. Just type the number above and set or unset it.

But do this after you already resurrect both old amber and helix/dome fossil. Then edit flag to get helix/dome fossil and get both kabuto and omanyte.

The overworld flag is use to get both the fossil at mt Moon. Just go back and the event which both you and the guy choosing the fossil will start again.

While the Choosing flag is for the resurrection process. Only one flag can be set. If you already choose Dome Fossil, then the flag will set for the prof at Cinnabar City to resurrect Dome Fossil only . Thus the problem with not recognising the Helix Fossil even it is in the bag .After resurrecting either Dome or Helix Fossil, unset/Set the event for the fossil which you didnt get. Then talk to the prof to get it resurrect.

The Starter Pokemon cheat works for getting both the dome and helix fossil but you have to be quick and prepare for a wild Pokemon to attack you just before you select the fossil... I suggest saving the game first it took me atleast 4 tries before I got it

Use the code 83000F9C 0000 when you get to the point that says would you like the dome/helix fossil say yes and immediately turn the cheat on quickly walk to the next fossil click on it then turn the cheat off and take it. Note ( this can also be used for all 3 starters)

Theres a cheat code if you look up fire red cheats and go through the list I don't have the code on hand atm