I don't think it would be necessary to spell more than usual. For most signs the non-dominant hand/arm is used as just a base or reference point for the dominant hand, or not at all. There are some signs that are two-handed "mirror-image" movements but most fluent signers can understand non-standard formations without much trouble. Think how many times people sign one-handed anyway--driving, eating, arm in a cast, holding a baby, carrying stuff, etc.
Certainly friends would get used to each others' signing styles. Even if part of a sign is "missing" the brain fills in the gaps, just like with reading fingerspelling.