Yes, it's possible for one to have two ISP's in one home. My mother enjoy's having her own DSL line provided by one ISP in our area, she's a work at home contractor programming for some database project or another, while my brother and I share a cable line provided by another ISP.
All three of our connections are online at the same time, and unless there's some kind of hidden ISP War behind the scenes, I don't believe our ISP's (Epix and Blue Ridge/PTD) mind very much either.
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