Hi, thank you for your question!
There is only one way I can think of to sync tasks between different Google Cals and Doit.im Cal.

For example, you have Google account 1 and 2.
1. You bind account 1 with Doit.im at first. Then you can see the tasks on Doit.im Cal those are from Google Cal 1.
2. Unbind account 1, and bind account 2, then you can see the tasks from both of the google cal 1 and 2 on Doit.im Cal.

what the inconvenience is when account 1 is bound with doit.im, if you delete any tasks in account 2 , those tasks you just deleted will not disappear in doit.im cal, which means you can only connect I google account with doit.im.

Can you understand what I mean?
If any problems, please feel free to contact us!
Thank you !