Given that ipv6 addressing often includes the outbound link (e.g. 26:01::02%eth0), I feel like it may not be required perhaps but also it could in some cases be helpful to have different devices for each.
It's all heavily virtualized nic stuff anyways so, I guess I'd ask: why not?
You need a multi-NIC VM for that? Isn't it commonplace in the IPv6 world for a single NIC to have multiple addresses? In fact I just checked: my computer at home has a single NIC but it has multiple addresses (both ULA and GUA) configured through SLAAC with privacy extension. In fact I count 14 addresses, excluding link-local.