I want to use 2 different SIM cards in a Nokia N900 phone, and each one of the cards have different contacts stored. This is because one SIM card is mine, and the other is my brother's.

We need that, when starting the phone with one of the SIMs, the device uses its contacts, and doesn't store the data in the phone. By this way, changing the operating SIM no other contacts remain and only the current SIM are to be used.

This is my question:
How can we use directly the contacts in the SIM card without copying them to the phone application/data?

Is the default phone application the only available for Maemo?
There aren't any alternative to make phone calls?

There isn't also any alternative for contacts app, to copy the contacts from phone to SIM card?

Have you tried
All Contacts>Groups>Sim card Contacts - select contact & dial

Ok, I've found and tried this path from the Contacts app. A little bit long this route. More large if you are in a call and wanna call another contact (+Menu -> Contacts -> ...)

Steps to reach a call:
1. From desktop, touch a Contacts icon
2. Touch out of the "Obtain contacts" dialog
3. Touch "All contacts"
4. Touch "Groups"
5. Touch "Sim card Contacts"
6. Touch a contact
7. Touch "Dial"

* In bold the steps caused by the lack of SIM card integration.
* If the user have selected wrong a contact, may not see the mistake, because in the phone only the number is shown (not the name)
* The calls history in the Phone App only show numbers, not names.