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.
No they do not need to be the same. I would place where the subs sound and measure best in your room. Under loudspeakers might not be the best sounding place for them but then again it could be, you will need to mess about with placement.
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thanks you two, that's interesting and helpful
by the way,i have just a two channel rig and no lfe output. does anyone have any insight into why that would be needed to use two different sized subs. thanks again.
another question, if using speaker level high pass connections, does one use only one set of speaker wires for the two subs, i.e., does one use only the left terminals on one sub, connecting to the amp's left speaker terminals, and repeat that for the right sub and amp terminals. or alternately, should one use two sets of wires thus employing all four binding posts on both subs and requiring doubling up two sets of wires on the amp terminals.

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thanks ericjcabrera,
that is how i envisioned it, but no retailing i've spoken to has agreed, though that could well be because they don't have experience with this approach and want to handle things according to their own sales approach.

re connections: i'm confused, one SET of speaker cables for each channel means two separate wires (each with a positive and negative sub-wire if you will) going to two terminals (i.e., positive and negative terminals on sub and amp). i guess you could attach two sub-wires to each terminal but that seems unnecessary, why not just take one wire from the SET, with a positive and negative sub-wire (sorry for the stupid neologism), and use that for the left and the other wire with its two subwires for the right. my apologies for the confusion. and thanks
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thanks for your response once again, unfortunately neither my amp (Trends 10.1 t-amp) or sub come with that or really any info, even if it did i don't think i'd know how to interpret it to determine such (in)compatibility on way or the other. i remain technically challenged. but thank you.