I started thinking about this question because of a favored combo of pokemon known as "Celetran" ; basically, Celebi and Heatran. People using the Shoddy Battle online pokemon battle simulator have been using this combo quite a bit lately. A couple of reasons for this immediately come to mind (although other reasons likely exist)...
-Celebi and Heatran resist attacks that would be Super Effective for the other.
-Celebi and Heatran have nice, bulky stats to take advantage of this.
But, what other pokemon might be pairable like this? Well, at a minimum, we can start with looking at the pokemon that simply resist weaknesses, just like the first Celetran bullet point. So, I've turn to my versatile pokemon program!
The results toss Uber and pokemon below UU. Its tiering is also kind of old so some pokemon that are Uber now might be in the results. sweatdrop
/Users/ekraft/pokemon > pokesearch -i resist -c "good_tier:tier!=uber && tier!=nu" good_tier -c "pairs:{x:pokemon}{size(goal(x))>0 && value(x,&good_tier) && covers(x,name) && covers(name,x)}" "size(goal(name))>0" "size(pairs)>0" -d pairs
id | name | pairs
-----------------------------------------------------------------------------------------------------------------------
6 | Charizard | quagsire
94 | Gengar | mightyena+absol+skuntank+umbreon+drapion
103 | Exeggutor | heatran
110 | Weezing | skuntank+drapion+mightyena+absol+umbreon
130 | Gyarados | magnezone+steelix
144 | Articuno | quagsire+swampert+whiscash+gastrodon
146 | Moltres | quagsire
149 | Dragonite | mawile+bastiodon+bronzong+probopass+lucario+metagross+aggron+registeel+magnezone+jirachi
171 | Lanturn | bronzong+skarmory
195 | Quagsire | moltres+scizor+bronzong+delibird+wormadam steel+charizard+forretress+articuno+skarmory
197 | Umbreon | mismagius+dusknoir+gengar+banette+weezing+rotom
205 | Forretress | gastrodon+kingdra+whiscash+flygon+swampert+garchomp+quagsire
208 | Steelix | pelipper+gyarados+mantine
212 | Scizor | garchomp+quagsire+flygon+kingdra+gastrodon+swampert+whiscash
225 | Delibird | gastrodon+whiscash+swampert+quagsire
226 | Mantine | steelix+magnezone
227 | Skarmory | lanturn+swampert+gastrodon+quagsire+whiscash
230 | Kingdra | forretress+wormadam steel+scizor+bronzong
251 | Celebi | heatran
260 | Swampert | scizor+wormadam steel+articuno+forretress+bronzong+delibird+skarmory
262 | Mightyena | weezing+dusknoir+banette+gengar+mismagius+rotom
279 | Pelipper | steelix+magnezone
303 | Mawile | salamence+dragonite+altaria
306 | Aggron | salamence+altaria+dragonite
330 | Flygon | forretress+jirachi+metagross+bronzong+scizor+wormadam steel
334 | Altaria | magnezone+mawile+probopass+bastiodon+jirachi+metagross+bronzong+aggron+registeel+lucario
340 | Whiscash | skarmory+bronzong+forretress+scizor+articuno+delibird+wormadam steel
354 | Banette | mightyena+umbreon+absol
359 | Absol | rotom+dusknoir+mismagius+gengar+banette+weezing
373 | Salamence | probopass+bastiodon+registeel+metagross+aggron+jirachi+lucario+bronzong+magnezone+mawile
376 | Metagross | altaria+flygon+dragonite+salamence
379 | Registeel | dragonite+altaria+salamence
385 | Jirachi | flygon+dragonite+salamence+altaria
395 | Empoleon | gliscor+rotom
411 | Bastiodon | dragonite+salamence+altaria
413 | Wormadam Steel | gastrodon+quagsire+flygon+kingdra+swampert+garchomp+whiscash
423 | Gastrodon | articuno+forretress+wormadam steel+bronzong+scizor+skarmory+delibird
429 | Mismagius | absol+skuntank+mightyena+drapion+umbreon
435 | Skuntank | weezing+rotom+gengar+mismagius
437 | Bronzong | garchomp+quagsire+kingdra+altaria+salamence+whiscash+dragonite+flygon+swampert+gastrodon+lanturn
445 | Garchomp | wormadam steel+bronzong+forretress+scizor
448 | Lucario | dragonite+altaria+salamence
452 | Drapion | gengar+rotom+weezing+mismagius
462 | Magnezone | altaria+dragonite+gyarados+pelipper+mantine+salamence
472 | Gliscor | empoleon
476 | Probopass | altaria+dragonite+salamence
477 | Dusknoir | umbreon+absol+mightyena
479 | Rotom | umbreon+empoleon+drapion+skuntank+absol+mightyena
485 | Heatran | exeggutor+celebi
