Yes, core1 power supply VDD is allowed to be connected to core0/platform power supply VDDC. The following is said in Design Checklist: "In single core devices, VDD can be connected to GND to save power, or else VDD should be connected from VDDC"
Here is a direct link to P1020/P1011 design checklist (application note AN4259):
https://www.nxp.com/webapp/Download?colCode=AN4259&location=null&fsrch=1&sr=1&pageNum=1&Parent_nodeI...
Have a great day,
Alexander
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