Using 5V from one PSU and 12V from another is OK to give you 17V. But you won't get 35A.
The 5V is usually around 30A but the 12V is only about 10-15A and that would restrict your setup to 17V 10-15A since you are connecting them in series.
Just choose the PSU with the highest amperage on the 12V. To get the most amps out of your setup.
Good luck
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Setup:
Dual Duron 1100 | Voodoo 3 2000 | Addtronics W8500(WTX) | Eheim 1250
| Car radiator | 2 Innovatech WB |
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