can i wire 2x 30amp double pole circuits to a 60 amp device
Globally, no.  You cannot "parallel" except in extremerly rare circumstances and with special equipment.  

But consult the documentation on that water heater.  You can do that if and only if the labeling or instructions say so, and by following the instructions.  This is codified in NEC 110.3B.  

(I'm hoping that the heater is actually two 30A heating elements internally, and some jumper straps could be removed to allow them to be powered separately). 

Given the annoyance of having to change the wiring like that, I would return that water heater and get one that takes 2-4 30A circuits. 

Also be aware you are halving the size of the on-demand heater.  It will give you dramatically less hot water.  A huge risk with on-demand water heaters is people failing to adequately size the heater, and then being disappointed with the results and blaming the heater.  This often leads to failed installations (people removing it in disappointment and having to buy another one).