ALTER TABLE TPersonRelation ADD CONSTRAINT FK_Person1ID FOREIGN KEY (Person1ID) REFERENCES TPerson(PersonID)

ALTER TABLE TPersonRelation ADD CONSTRAINT FK_Person2ID FOREIGN KEY (Person2ID) REFERENCES TPerson(PersonID)

create table tperson(personid int primary key, personname sysname)
go
create table tpersonrelation(person1 int, person2 int)
go
insert tperson (personid, personname) values(1, 'aaa')
go
--2 doesn't exist in tperson, but we can add this because there is no FK constraint yet
insert tpersonrelation(person1, person2) values(1, 2) 
go
alter table tpersonrelation add constraint fk_person1 foreign key(person1) references tperson(personid)
go
--this statement fails because of the insert above
alter table tpersonrelation add constraint fk_person2 foreign key(person2) references tperson(personid)
go
drop table tpersonrelation
drop table tperson