I think MAC addresses are assigned from lowest to highest address as each VLAN is created. So if you created VLAN1 and then VLAN2 on one slot, but created VLAN2 then VLAN1 on the second slot, the MAC addresses would be reversed between the two. You can see this on a single slot if you create, delete and then recreate two VLANs in opposite order:
b vlan VLAN1 {interfaces 2.1}
b vlan VLAN2 {interfaces 2.2}
b vlan|grep ^VLAN
VLAN VLAN1 tag 4094 00:017:60:B3:83 MTU 1500
VLAN VLAN2 tag 4093 00:017:60:B3:84 MTU 1500
b vlan all delete
b vlan VLAN2 {interfaces 2.2}
b vlan VLAN1 {interfaces 2.1}
b vlan|grep ^VLAN
VLAN VLAN1 tag 4093 00:017:60:B3:84 MTU 1500
VLAN VLAN2 tag 4094 00:017:60:B3:83 MTU 1500
Chris's suggestion of using MAC masquerading should be a good way to avoid this as it allows you to manually specify a custom address to a VLAN or all VLANs on both units.
Or you could ensure the VLANs are created in the same order. You could also manually clear the ARP tables of neighboring switches to avoid this when booting between slots.
Aaron