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.
For HT, they do not necessarily need to be the same. I used to have one ten inch sub and another twelve inch sub for my recently dismantled HT. They both complimented each other by filling in what the other didn't. And they were placed in different locations too.

OTOH, for two channel / stereo, it depends on what outputs you are using. I would recommend identical subs if feeding one from left and the other from right outputs respectively. If using an LFE output, then it shouldn't matter.
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.

In my experience, two subs of different sizes actually sounded better than using only one of either sub. The smaller one gave my system the PUNCH, while the larger one filled in some serious RUMBLING.
Speaker level connections: only one set of cables necessary for each channel respectively. Although most sub frequency is non directional, keeping them separate should still give you an overall better sound stage.
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

Yeah, if using speaker outs, just one cable (+&-) to drive one channel of each sub. In most subs, the left input is configured for mono, while the right input, when used in conjunction with the left, is differential gain. (retains both L+R and L-R signals.)

One other caution; make sure your amp is compatible with the given subs input. If it does not have common ground, (like my current amps are differentially balanced) it could get damaged if connected to a common ground sub speaker input. Check out the subwoofer specs and your amps specs too before making the connection!

The safest way is to use low level single ended or balanced ic's and run w/ the subs inputs and outputs. That way you can have the subs crossover separate the lows and the highs.
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.