Merge two row records I have 2 tables 

Person Table 

PersonID   Firstname LastName
1              a         b
2              c         d
3              e         f
4              g         h
5              i         j


Incident Table 

InicdentID PersonID 
1             1
1             2
2             3
2             4
3             5


I want to STUFF Name with semicolumn for one Incident

IncidentID Name 
1            ab;cd
2            ef;gh
3            ij
Try this

create table #person (
PersonID int,
FirstName varchar(5),
LastName varchar(5)
)

create table #incident (  
IncidentID int,
PersonID int
)

insert into #person values (1,'a','b')
insert into #person values (2,'c','d')
insert into #person values (3,'e','f')
insert into #person values (4,'g','h')
insert into #person values (5,'i','j')

insert into #incident values (1,1)
insert into #incident values (1,2)
insert into #incident values (2,3)
insert into #incident values (2,4)
insert into #incident values (3,5)


 select t.IncidentID,
   stuff(
    (select ';' + p.firstname + p.lastname
      from #incident i
      join #person p
        on i.personid = p.personid
      where i.incidentid = t.incidentid
     for xml path('')),
     1,
     1,
     ''
   )
 from #incident t
group by t.IncidentID


Documentation on stuff() command