Input (you choose the data structure that you take in):
   Server 1              
 Time   Users          
   0      5                   
   3      10               
   15     2                
   25     15                


Server 2
Time   Users
3         10
10        3
12        8
20        8
30        5

Add the time series of both servers. Note the users is a step function.
Eg for server1  : time 0 Users 5
							time 1 users 5
							time 2 users 5

Output (whatever data structure you ended up choosing):
Time   Users
   0      5
   3      20
   10     13
   12     18
   15     5
   20     10
   25     23
   30     20