Currently with the v2 API registering different API keys so each one can have its own redirect URI would be a viable solution. You could also use just one redirect URI and from there link to each of the individual apps.
When we release our v3 API it will support multiple redirect URIs. We do not have an exact date on the release, but we are hoping it will be within the next few months. Keep an eye out on these forums; also watch developer.constantcontact.com for an update.
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