This is impossible if you extracted via part1 using proper software. However, if you manually extracted part1 and then part2 into separate folders, you will have split data. Delete everything and extract only from the first volume again .
filename.rar (part 1), filename.r00 (part 2), filename.r01 (part 3), etc. Step-by-Step Extraction Guide 1. Prepare Your Files how to extract multi volume rar files
: If you have part1 and part3 , but part2 is missing, the extraction will fail. This is impossible if you extracted via part1
If you try to extract just the first file, will you get everything? What if one file is missing? Why do these archives exist in the first place? filename.r00 (part 2)