Well the only way to fix your problem would be to root/hack/flash the Streak. Put CyanogenMod (this is a custom ROM). Put the SIM card into the Streak (this is now your primary phone).

Turn on the N900 without a SIM card (and use it basically as an Internet Tablet/MID). When you want 3G on your N900, turn on the tether from the Streak (over Wifi).
With the N900 scan for Wifi signals, find the Streak, Enter the password and Connect.

Now if someone calls you, your Streak will ring.
You can use the 3G data/internet on both phones simultaneously (but it might be a little slower).
Be careful of your battery on both devices (especially the Streak) since tethering tends to consume plenty, but if your Streak runs out of battery before the N900 just swap the SIM card into the N900 and keep on going.

I'm doing the same thing with my laptop and my Galaxy S (although I don't have to pay the carrier fees/flash a custom ROM).