using 2 subs: do they need to be the same?

i'm wondering about adding a second sub. do they generally need to be the same size woofer, make, model, etc. as long as they are playing below 80hz? as far as placement goes, can they be placed directly under the monitors (which would be on top of specially designed stands to accommodate this), as if they were part of a single pair of floor standers. i have read here that they should be asymmetrically set up, but how is there use different than a single pair of speakers with integral powered subs, at least as far as worrying about troom modes go. as always, thanks for any advice.