Short answer, this cannot be done in IPFire.
Long answer, IPfire supports only multiple dial-up profiles and it will automatically switch from one profile to the other one, if there is a failure. If you have two providers, you can write two different dial-up scripts and IPFire will switch over if needed, in other words If one fails, IPFire will call the other one. However, you must have only ONE modem coming to your ethernet card attributed to the red zone.
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