Este é um tiro no escuro porque realmente não conhecemos seus dados, mas se você gostar de sua consulta existente, pode ser possível simplesmente usá-la duas vezes, mas com uma cláusula where como esta:
SELECT
-- Common stuff?
ISNULL(Sales.ActivityId, Other.ActivityId), ISNULL(Sales.ScheduledStart, Other.ScheduledStart), ISNULL(Sales.OwnerIdName, Other.OwnerIdName), ISNULL(Sales.AccountIdName, Other.AccountIdName), ISNULL(Sales.new_ContactPersonName, Other.new_ContactPersonName), ISNULL(Sales.[Subject], Other.[Subject]), ISNULL(Sales.new_ColderNotes, Other.new_ColderNotes)
-- Sales stuff?
, Sales.AccountId, Sales.ContactId, Sales.SystemUserId, Sales.OptionalOwner, Sales.OptionalContact, Sales.OptionalAccount, Sales.PartyId, Sales.ParticipationTypeMask, Sales.RequiredContact, Sales.RequiredAccount, Sales.RequiredOwner, Sales.new_BusinessUnit
-- Other Stuff?
, Other.AccountId, Other.ContactId, Other.SystemUserId, Other.OptionalOwner, Other.OptionalContact, Other.OptionalAccount, Other.PartyId, Other.ParticipationTypeMask, Other.RequiredContact, Other.RequiredAccount, Other.RequiredOwner, Other.new_BusinessUnit
FROM
(
SELECT DISTINCT
Appointment.ActivityId, Appointment.ScheduledStart, Appointment.OwnerIdName, Contact.AccountIdName, Appointment.new_ContactPersonName,
Appointment.Subject, Appointment.new_ColderNotes, Account.AccountId, Contact_1.ContactId, SystemUser.SystemUserId, SystemUser.FullName AS OptionalOwner,
Contact_1.FullName AS OptionalContact, Account.Name AS OptionalAccount, ActivityParty.PartyId, ActivityParty.ParticipationTypeMask,
Contact_1.FullName AS RequiredContact, Account.Name AS RequiredAccount, SystemUser.FullName AS RequiredOwner, Account.new_BusinessUnit
FROM
Contact AS Contact_1 RIGHT OUTER JOIN
Account RIGHT OUTER JOIN
SystemUser RIGHT OUTER JOIN
Appointment INNER JOIN
ActivityParty ON Appointment.ActivityId = ActivityParty.ActivityId ON SystemUser.SystemUserId = ActivityParty.PartyId ON Account.AccountId = ActivityParty.PartyId ON
Contact_1.ContactId = ActivityParty.PartyId LEFT OUTER JOIN
Contact ON Appointment.new_ContactPerson = Contact.ContactId
-- Limit this part to Sales?
WHERE ParticipationTypeMask BETWEEN 1 AND 3
) as Sales
FULL OUTER JOIN
(
SELECT DISTINCT
Appointment.ActivityId, Appointment.ScheduledStart, Appointment.OwnerIdName, Contact.AccountIdName, Appointment.new_ContactPersonName,
Appointment.Subject, Appointment.new_ColderNotes, Account.AccountId, Contact_1.ContactId, SystemUser.SystemUserId, SystemUser.FullName AS OptionalOwner,
Contact_1.FullName AS OptionalContact, Account.Name AS OptionalAccount, ActivityParty.PartyId, ActivityParty.ParticipationTypeMask,
Contact_1.FullName AS RequiredContact, Account.Name AS RequiredAccount, SystemUser.FullName AS RequiredOwner, Account.new_BusinessUnit
FROM
Contact AS Contact_1 RIGHT OUTER JOIN
Account RIGHT OUTER JOIN
SystemUser RIGHT OUTER JOIN
Appointment INNER JOIN
ActivityParty ON Appointment.ActivityId = ActivityParty.ActivityId ON SystemUser.SystemUserId = ActivityParty.PartyId ON Account.AccountId = ActivityParty.PartyId ON
Contact_1.ContactId = ActivityParty.PartyId LEFT OUTER JOIN
Contact ON Appointment.new_ContactPerson = Contact.ContactId
-- Limit this part to Other?
WHERE ParticipationTypeMask BETWEEN 4 AND 6
) AS Other ON Sales.ActivityId = Other.ActivityId -- More cols for join?
