Despite the fact this is off-topic, I'm going to give this a formal solution just to get it out of the list of Unanswered Questions.
If you insist on doing this in a cmd window then you have 2 options.
1. Follow the advice given by Sergey... Either have a buffer file to include in the COPY command or use the technique provided on the link.
2. Use TYPE instead of COPY (which is actually used on the link in SA's comment)
TYPE t1.csv > t3.csv
ECHO. >> t3.csv
TYPE t2.csv >> t3.csv
Alternatives have already been mentioned by Richard - use Copy+Paste in something like Notepad, or use Excel to load the data.
If this is going to happen often then write a program to merge the text from the files.