Does this mean that both users' portfolios will have information about this transaction?
The two miners will see their own coinbase releases in their wallet software, yes. However, the two coinbase transactions will be separated.
If so, what will happen when both users go quickly to the Bitcoin ATM and withdraw the fiat currency from this reward?
They can not do that. Coinbase exits are immature, and can not be spent, for 100 blocks. After 100 blocks, a fork should ideally be solved already. If it is not resolved, only the minor located on the same side of the fork as the ATM operator will be able to access his coins because the ATM will not have any knowledge of the parts on the other side of the fork.