They don't have to be, but you'll have an easier time integrating if they are.
If you can't go identical, or don't want to, then the next best thing is subs that are tonally the same, so different models from the same manufacturer or using the same driver.
If you can't do that (or don't want to), then stick with the same type of subs (sealed OR ported), don't mix types. You can integrated different types of sub, but it's much harder to do well.
I've mixed four subs in my room. Two of one model from one manufacturer, a different model from the same manufacturer (PSA), and a sealed sub from a different manufacturer (Velodyne). They integrate well and fairly easily.
OK I've read your thread and some others... Lot of stuff to digest here! I'm a bit new into that. I've beeing using REW a couple times to measure my room but I'm no specialist into what to do to improve and correct etc. I've tried a couple times to go into manual settings of my AVR to play with Freq, Gain and Q but I've been unsucceful yet. I will need some help! But prior to ask for your help and start a new thread, please tell me what would YOU need as information so you won't have to ask many times! Here what I think could be usefull : AVR, speakers type, room size, accoustic treatment, REW measurements (but WHICH? : FL and FR speakers seperatly, both, with or without subs, all together???). Anything else?
And BTW : I bought 2 new subs! I did go with 2 Klipsh R-120SW. I know it's not Nirvana ;-) but for the budget, I've seen a huge diffence!!!
Thanks in advance! Looking forward to test things after your recommandations!