buying pair graphics is better

[JOHN PLAYER]

i have already a gtx 970 but i need to update
shoud i buy pair of gtx 1070 or a single gtx 970 ..
i want more cuda cores in 1500$

 

[Isokolon]

for CUDAs (I assume you don't game but need it for editing tasks) 2x 980Ti have actually more CUDAs than the 1070s. If your PSU can handle it.

 

[JOHN PLAYER]

mainly render task handle
gaming also
is quadro graphics a solution

 

[lordmogul]

For redering tasks either go with a Quadro or switch to AMD.
For gaming going with 1070s will give you more performance than with 970s
Also you would have more performance if you don't use SLI (microstuttering, games without SLI support, etc)
And 8 GB per card is just way more than 3.5 GB per card.