Yes, you can append them together ...
| inputcsv myfirstfile.csv | inputcsv append=t mysecondfile.csv
... here's a way to simulate those results ...
| makeresults count=24 | streamstats count as monthno
| eval _time=relative_time(_time,"-3y@y-1mon+".monthno."mon@mon")
| eval usage=100*(50+ random()%50+random()%50)
| rename COMMENT as "The above just makes 24 months of data"
...and then either ...
1) make a 1-year graph with two series, one for each year...
| eval Year = strftime(_time,"%Y")
| eval Month = strftime(_time,"%m %b")
| eval _time = if(Year="2014", relative_time(_time,"+1y"),_time)
| chart sum(usage) over Month by Year
... or ...
2) make a single 2-year graph ...
| timechart sum(usage) by Year
| where _time=86400*floor(_time/86400)
The last part is to get rid of some garbage records that timechart has been adding onto the end lately
