Perhaps I should clarify. There simply is not enough time. The demands of working one lab are rigorous enough. As I said before, I spend on average 40-50 hours a week working my lab, meetings, training (i.e. TEM takes a ****-ton of training and practice) or writing papers. This does not include reading papers at home and what not. Days go by fast and when it's crunch time you can easily hit 70 hours a week.
When you have courses, volunteering commitments, and most importantly your health/sleep/diet to consider as well...you can easily see how time becomes a commodity. Now 2 labs, while I cannot say it is impossible, I would not recommend it based on my personal experiences
in 1 lab.
Overall, I would recommend 1 lab and focusing on quality
, rather than quantity. If you can get one shining recc, but more importantly, a great experience learning and being exposed to the climate of research, then that is much more worth it than putting down 2 labs you were in maybe 10 hrs a week collectively and not getting anything worthwhile done, or learning much. People rarely want to train someone for free who is just going to leave right afterwards. You need some output into the lab, and for that you need time. Just my 2 cents.
edit: by the way most undergrads in my lab do not put in 40 hrs a week so that's probably an overestimate if you want to be general. I just like to spend my free time in lab. However, you can see a stark contrast in the levels of output between myself and people who devote less time to research. You get what you put in.
Finally, I want to emphasize that this is just 1 person's opinion so take it with a grain of salt. But if you think about it, let's say you want to excel at sports. Someone trying to play both golf and basketball well will have less time to do each compared to someone who did golf exclusively or basketball exclusively.