From your post, it sounds like you are going to put two dragons in a 75 gal tank no matter what our opinion is, so why ask for it?
Anyway, will give my view on it anyway. As a 75 gal tank is only 48"x18"x21" it is not even the recommended minimum size for a single adult dragon (recommended mim size is 48"x24"x24"
thus it is deffinitely too small for two adult dragons.
I can understand space limitations but it would be better to build two tanks and set them one onto of the other, thus giving that cage size or better yet, the actual recommended cage size minimum of 48"x2"x2" for each single dragon. This way, you have more effectively used the space available (one can be directly on the floor, the second sitting ontop of the other, thus the over all space required would be 48"x48"x24"
. Also, dragons do not always get along.
Even when raised together from a young ages, two females can suddenly decide they do not like eachother once reaching adulthood or even after a few years of living peacefully together. This may never happen but there is that chance. Housing two males together is almost guarenteed to result in one or both being injuried in fights and the loss of one or both from injuries and/or stress. A male and female pair means dealing with clutches of eggs, extra calcium and food for the female, most likely having her bred too young and over bred and basically stressed out from him trying to mate with her as often as he can. Even then, they can fight and cause injuries, the female could stop eating or suffer from calcium deficiences due to shelling multiple clutches of eggs each year and generally shortening her lifespan.
Common problems is one of the pair (be it two females, male/female, male/male) is one will be dominate and eat most or all of the food, take the best basking spots (either by chasing the other away and not allowing it to bask, or 'stacking' ontop of her, blocking the heat/uvb from reaching the bottom dragon), stressing the other dragon and even causing stress to the dominant dragon as now he/she has to be viligent on keeping the submissive dragon in his/her place. Fighting can cause loss tails due to being bitten off, loss toes for the same reason, other body wounds that can become infected and further weakening the submissive dragon. THe submissive dragon most likely will hide, staying as much out of the way of the dominant dragon as the cage allowes, not eating properly or enough, not getting sufficient basking heat to digest food nor access to sufficient amounts of UVB lights, calcium, V3 etc which eventually leads to the dragon's death.
Other common problems is spread of parasites and other communicable diseases. Stress often causes inbalances of gut flora, allowing harmful bacterias/parasites to flare up (such as coccidia) and further stressing the body of the dragon(s). ALso being in the same environment, the dragons will most likely cross-contaminate eachother resulting in both needed to be treated even if only one shows signs of having parasites. Also that much harder to determine which dragon has parasites from stool samples unless you actually see the dragon poop. Other diseases such as adrenovirus can also be spread in close proximity, external parasites etc.
But despite the above risks and dangers of housing two dragons together, especialy in a cage too small for a single adult dragon, you choose to do so, then be prepared to do alot of extra work to ensure your dragons thrive as well as they are able. Make sure both dragons are completely parasite/disease free before introducing them to eachother (vet checks, fecals, etc). Set up several different basking spots and feeding areas, to ensure both dragons have a chance to eat and bask normally. Make sure the dragons are either both female or male female (but only put together after the female is at least 15 months old, to breed. If you do not want to breed, then purchase two female dragons. Note that dragons are difficult to sex under 6 months of age, so mistakes can happen if you buy two dragons marked as female under 6 months by breeders with experience sexing younger dragons.
Treat any and all minor injuries with topical antibiotics. If the wounds are deep, take to a vet for treatment (most likely combination of topical and oral/injectable antibiotics). If you have babies and one grows faster than the other, you may need to separate till the smaller one catches up in size. Dragons housed together who are noticeable of different sizes risks the smaller dragon being eaten by the larger dragon, especially if a great difference in size. If only a small margin in size differences, larger dragon could dominate the smaller one and prevent it from eating etc, thus preventing it from growing and thriving.
If you are prepared for the added risks and willing to take the dragons to a vet as needed and understand a small cage can put as much stress on a dragon as having a cage mate it doesn't get along with, could result in dragons that do not thrive, grow as big, etc. thus resulting in the loss of one or both dragons over time, then go ahead with your plans. However, expect alot more flack here if you post for help about problems you are having later on down the road due to starting off with a cage too small for two dragons. Not giving you a hard time just making you aware of possible difficulties down the road. OF course any posts attacking you directly will be removed as they violate TOS, it doesn't mean they won't come up and be noticed before removed by a forum Mod. Feel free to use the 'report abuse' link if such messages are noticed by you or anybody else for that matter.
But, I suspect most wil recommend separating the dragons, increasing the size of the cage, vet treatment etc, if problems occur from your planned setup.
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PHLdyPayne