This is a great question and really is not an uncommon question.
As a matter of fact many worm sellers will sell to buyers this way as the buyers are undecided in which species they want to raise.
The most common composting worm species that are mixed are the Eisenia Fetida (red wiggler) and the Eisenia Hortensis (European Nightcrawler).
The reason behind mixing them is because the worm farmer likes the productivity of the red wiggler and also likes the fatness of the European Nightcrawler for fishing.
So if you want to mix these then they will certainly get along but I would stress that over time the red wiggler will out populate the European Nightcrawler.
The red wiggler is a "super worm" when it comes to eating, procreating and creating lots of worm castings. So keep this in mind.
Thanks for your question and I hope this helps if not then reply again to this post.
~Pauly
Apr 08, 2013 Rating
worm grower by: txfshrmn
4/8/2013 0900
hello, my name is Chuck and it seems self explanatory to me not to mix worm breeds
TY
Apr 08, 2013 Rating
worm grower by: Chuck
0930
its me agn, i just started my farm.i lv in N Tx and
i an retired, but i needed a hobby. i still want to
raise crayfish (Krayfish Korner). i know alot abt the
outdoors than most, eatable foods as well.