Generating +65 V (positive) and -65 V (negative) voltages from two 12 V batteries using cheap step-up converters?
i got the solution for my problem:
Bat1 + -&gt; + converter 1 +65 -&gt; amp +
Bat1 - -&gt; - converter 1 0   -&gt; amp GND
Bat2 + -&gt; + converter 2 +65 -&gt; amp GND
Bat2 - -&gt; - converter 2 0   -&gt; amp -

So don't cross the battery output before it goes into the converters - For everyone who has the same question ;)