no they don't have to be, just like left and right speakers don't have to be the same.
ideally you do want them to be the same though so they will be producing the same output. i don't think i've ever seen someone run 2 different subs though, for car or home audio.
I have seen plenty of people running different subs, but you need to do it properly. Typically in a multi-sub setup where there are different subs, either the receiver has multiple sub outputs which send different frequency ranges to the different subs (i.e. one sub is a 8" or 10" sub with fast response for the higher frequencies of the low frequency response, and the other sub(s) is a 15" or 18" sub that handles the real low bass). If you do not have a receiver/pre-processor that handles the multiple sub output ranges, an external crossover is used to perform the same thing to split the overall frequency response between the multiple subs.
LFE response on most setups can easily still be within the human voice range. As such, the benefits of having a smaller sealed 8" or 10" sub which has extremely fast, accurate response can drastically improve the sound quality of your speaker setup, and using the larger 15" and larger to simply handle the 40 Hz and lower response will keep the smaller sealed sub from being overly stressed attempting to give you the boom that you want out of explosions and other LFE effects in movies.
Once you are running multiple subs, you do need to pay a lot of attention to distances and phase. Most crossovers act over a two octave range where the signal overlaps between the multiple devices (i.e. a 80 Hz crossover means that frequencies between 40Hz and 160 Hz are played on both devices typically with a 6 to 8 db drop curve per octave, so that at 80 Hz both devices are playing at the same at negative 6 to negative 8 db from the reference level of the signal so that the combined output of both speakers is reference level of the signal (two speakers playing the same thing will typically produce a 6 to 8 db boost to the level of the combined output). Due to this combining of the waveforms to generate the proper output levels, placement of the subs should ideally be equi-distant from the listening position(s) and the phase set perfectly between them to account for differences in length of speaker wire, processing/response time, and differences in angle/elevation of the different sub's woofers in relation to the ears listening to them. The last thing you want to have happen is destructive interference from the waves being out of phase with each other when they reach the ear listening to the sound. At this point, it is highly recommended that you consult with professionals for proper setup and design.