Hello Niranjan,

you can achieve load-sharing on per vlan basis.

there are two tools you can use, one or both, to achieve this.

First of all you can make core1 root bridge for a subset of existing vlans and core2 root bridge for the remaining vlans.

This should provide STP forwarding state for first set of vlans on direct link to core1 and STP forwarding state for the second set of vlans on link towards core2

spanning-tree root primary macro accepts a range of vlans expressed as XX,YY,Z1-ZN, you need to use ',' as separator without spaces in the middle.

another tool that can help is

on the link interface configuration

spanning-tree pervlancost Vlan-range were you can increate the STP cost on the access switch uplinks for the vlans you want to be in STP forwarding state on the other uplink. You then do the opposite on the other uplink

Hope to help

Giuseppe