There is a script file in your Spike7 script folder (usually in My Documents), that you can use to merge two files together, with a combination of chooosing which channels to import and others. There is a newer version on the website (https://ced.co.uk/downloads/scriptspkedit
), that might do more for you. You should first import your two separate edf files into Spike2 using the File->Import facility, creating two smr files, which can be then merged together using the script mentioned above.
I hope this helps.