YES! It is possible to obtain both 
Here is how:
1) This must be done BEFORE enterng the Spire for the 1st time.
2) Go and join the temple of shadows
3) Sacrifice atleast 4 people (I read that, personatly i did 12. 4 people, 4 that i married in the temple and 4 monks-almost 1000 bad points) [Note: make them follow you but fast travel because it's a lot easier-they follow even like that]
4) Go and do the Defender of the light quest (NOT oakfield massacre) in order to save the temple of light
5) That's it! Even though it say's that the temple of the shadows will never recover it semi-does (till now i haven't seen people BUT the building is intact, you can still sacrifice AND buy the building)
6) Go to the Spire
7) Then you can simply buy both temples (to me it was 96-97.000 for the light (975 income) and 100.000 for the shadows (1.000 income)
Note: You cannot sell none of those buildings!
I know that normally it's not possible but this is done perhaps because you make the temple of shadows strong enough so it can be sustaind
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I didn't saw anything like this here so I said to write it down for you guys
It would work well as a sticky too
Here is how:
1) This must be done BEFORE enterng the Spire for the 1st time.
2) Go and join the temple of shadows
3) Sacrifice atleast 4 people (I read that, personatly i did 12. 4 people, 4 that i married in the temple and 4 monks-almost 1000 bad points) [Note: make them follow you but fast travel because it's a lot easier-they follow even like that]
4) Go and do the Defender of the light quest (NOT oakfield massacre) in order to save the temple of light
5) That's it! Even though it say's that the temple of the shadows will never recover it semi-does (till now i haven't seen people BUT the building is intact, you can still sacrifice AND buy the building)
6) Go to the Spire
7) Then you can simply buy both temples (to me it was 96-97.000 for the light (975 income) and 100.000 for the shadows (1.000 income)
Note: You cannot sell none of those buildings!
I know that normally it's not possible but this is done perhaps because you make the temple of shadows strong enough so it can be sustaind
--------------------------------------
I didn't saw anything like this here so I said to write it down for you guys
It would work well as a sticky too